Reland "[Modules] Fix using va_list with modules and a precompiled header. (#100837)"

Fix the false warning
> incompatible pointer types passing 'va_list' (aka '__builtin_va_list')
to parameter of type 'struct __va_list_tag *'
[-Wincompatible-pointer-types]

The warning is wrong because both in the function declaration and at the
call site we are using `va_list`.

When we call `ASTContext::getBuiltinVaListDecl` at a specific moment, we
end up re-entering this function which causes creating 2 instances of
`BuiltinVaListDecl` and 2 instances of `VaListTagDecl` but the stored
instances are unrelated to each other because of the call sequence like

    getBuiltinVaListDecl
      CreateX86_64ABIBuiltinVaListDecl
        VaListTagDecl = TagA
        indirectly call getBuiltinVaListDecl
          CreateX86_64ABIBuiltinVaListDecl
            VaListTagDecl = TagB
          BuiltinVaListDecl = ListB
      BuiltinVaListDecl = ListA

Now we have `BuiltinVaListDecl == ListA` and `VaListTagDecl == TagB`.

For x86_64 '__builtin_va_list' and 'struct __va_list_tag *' are
compatible because '__builtin_va_list' == '__va_list_tag[1]'. But
because we have unrelated decls for VaListDecl and VaListTagDecl the
types are considered incompatible as we are comparing type pointers.

Fix the error by creating `BuiltinVaListDecl` before
`ASTReader::InitializeSema`, so that during
`ASTContext::getBuiltinVaListDecl` ASTReader doesn't try to de-serialize
'__builtin_va_list' and to call `ASTContext::getBuiltinVaListDecl`
again.

Reland with the requirement to have x86 target to avoid errors like
> error: unable to create target: 'No available targets are compatible
with triple "x86_64-apple-darwin"'

rdar://130947515

Author: vsapsai

clang/lib/Sema/Sema.cpp

@@ -310,6 +310,13 @@ void Sema::addImplicitTypedef(StringRef Name, QualType T) {
 }
 
 void Sema::Initialize() {
+  // Create BuiltinVaListDecl *before* ExternalSemaSource::InitializeSema(this)
+  // because during initialization ASTReader can emit globals that require
+  // name mangling. And the name mangling uses BuiltinVaListDecl.
+  if (Context.getTargetInfo().hasBuiltinMSVaList())
+    (void)Context.getBuiltinMSVaListDecl();
+  (void)Context.getBuiltinVaListDecl();
+
   if (SemaConsumer *SC = dyn_cast<SemaConsumer>(&Consumer))
     SC->InitializeSema(*this);
 

clang/test/Modules/builtin-vararg.c

@@ -0,0 +1,84 @@
+// Check how builtins using varargs behave with the modules.
+
+// REQUIRES: x86-registered-target
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -triple x86_64-apple-darwin \
+// RUN:   -fmodules -fno-implicit-modules -fbuiltin-headers-in-system-modules \
+// RUN:   -emit-module -fmodule-name=DeclareVarargs \
+// RUN:   -x c %t/include/module.modulemap -o %t/DeclareVarargs.pcm \
+// RUN:   -fmodule-map-file=%t/resource_dir/module.modulemap -isystem %t/resource_dir
+// RUN: %clang_cc1 -triple x86_64-apple-darwin \
+// RUN:   -fmodules -fno-implicit-modules -fbuiltin-headers-in-system-modules \
+// RUN:   -emit-pch -fmodule-name=Prefix \
+// RUN:   -x c-header %t/prefix.pch -o %t/prefix.pch.gch \
+// RUN:   -fmodule-map-file=%t/include/module.modulemap -fmodule-file=DeclareVarargs=%t/DeclareVarargs.pcm \
+// RUN:   -I %t/include
+// RUN: %clang_cc1 -triple x86_64-apple-darwin \
+// RUN:   -fmodules -fno-implicit-modules -fbuiltin-headers-in-system-modules \
+// RUN:   -emit-obj -fmodule-name=test \
+// RUN:   -x c %t/test.c -o %t/test.o \
+// RUN:   -Werror=incompatible-pointer-types \
+// RUN:   -fmodule-file=%t/DeclareVarargs.pcm -include-pch %t/prefix.pch.gch \
+// RUN:   -I %t/include
+
+//--- include/declare-varargs.h
+#ifndef DECLARE_VARARGS_H
+#define DECLARE_VARARGS_H
+
+#include <stdarg.h>
+
+int vprintf(const char *format, va_list args);
+
+// 1. initializeBuiltins 'acos' causes its deserialization and deserialization
+//    of 'implementation_of_builtin'. Because this is done before Sema initialization,
+//    'implementation_of_builtin' DeclID is added to PreloadedDeclIDs.
+#undef acos
+#define acos(__x) implementation_of_builtin(__x)
+
+// 2. Because of 'static' the function isn't added to EagerlyDeserializedDecls
+//    and not deserialized in `ASTReader::StartTranslationUnit` before `ASTReader::InitializeSema`.
+// 3. Because of '__overloadable__' attribute the function requires name mangling during deserialization.
+//    And the name mangling requires '__builtin_va_list' decl.
+//    Because the function is added to PreloadedDeclIDs, the deserialization happens in `ASTReader::InitializeSema`.
+static int __attribute__((__overloadable__)) implementation_of_builtin(int x) {
+  return x;
+}
+
+#endif // DECLARE_VARARGS_H
+
+//--- include/module.modulemap
+module DeclareVarargs {
+  header "declare-varargs.h"
+  export *
+}
+
+//--- resource_dir/stdarg.h
+#ifndef STDARG_H
+#define STDARG_H
+
+typedef __builtin_va_list va_list;
+#define va_start(ap, param) __builtin_va_start(ap, param)
+#define va_end(ap) __builtin_va_end(ap)
+
+#endif // STDARG_H
+
+//--- resource_dir/module.modulemap
+module _Builtin_stdarg {
+  header "stdarg.h"
+  export *
+}
+
+//--- prefix.pch
+#include <declare-varargs.h>
+
+//--- test.c
+#include <declare-varargs.h>
+
+void test(const char *format, ...) {
+  va_list argParams;
+  va_start(argParams, format);
+  vprintf(format, argParams);
+  va_end(argParams);
+}