[windows] Always pass libcall fp128 arguments indirectly

LLVM expects `__float128` to be both passed and returned in xmm
registers on Windows. However, this disagrees with the Windowx x86-64
calling convention [1], which indicates values larger than 64 bits
should be passed indirectly.

Update LLVM's libcall calling convention to pass `fp128` directly.
Returning in xmm0 is unchanged since this seems like a reasonable
extrapolation of the ABI.

With this change, the calling convention for `i128` and `f128` becomes
the same.

This corresponds to the frontend change in [2].

[1]: https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170
[2]: #115052

Author: tgross35

llvm/lib/Target/X86/X86CallingConv.td

@@ -639,8 +639,9 @@ def CC_X86_Win64_C : CallingConv<[
   // 512 bit vectors are passed by pointer
   CCIfType<[v64i8, v32i16, v16i32, v32f16, v32bf16, v16f32, v8f64, v8i64], CCPassIndirect<i64>>,
 
-  // Long doubles are passed by pointer
+  // Float types larger than 64-bits (long double and fp128) are passed by pointer
   CCIfType<[f80], CCPassIndirect<i64>>,
+  CCIfType<[f128], CCPassIndirect<i64>>,
 
   // If SSE was disabled, pass FP values smaller than 64-bits as integers in
   // GPRs or on the stack.