[libc++] Rename __bit_reference template parameter to avoid conflict (#80661)

As of 4d20cfc, `__bit_reference`
contains a template `__fill_n` with a bool `_FillValue` parameter.

Unfortunately there is a relatively widely used piece of scientific
software called NetCDF, which exposes a (C) macro `_FillValue` in its
public headers.

When building the NetCDF C++ bindings, this quickly leads to compilation
errors when the macro interferes with the template in `__bit_reference`.

Rename the parameter to `_FillVal` to avoid the conflict.

Author: DimitryAndric

libcxx/include/__bit_reference

@@ -173,7 +173,7 @@ private:
 
 // fill_n
 
-template <bool _FillValue, class _Cp>
+template <bool _FillVal, class _Cp>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
 __fill_n(__bit_iterator<_Cp, false> __first, typename _Cp::size_type __n) {
   using _It            = __bit_iterator<_Cp, false>;
@@ -185,7 +185,7 @@ __fill_n(__bit_iterator<_Cp, false> __first, typename _Cp::size_type __n) {
     __storage_type __clz_f = static_cast<__storage_type>(__bits_per_word - __first.__ctz_);
     __storage_type __dn    = std::min(__clz_f, __n);
     __storage_type __m     = (~__storage_type(0) << __first.__ctz_) & (~__storage_type(0) >> (__clz_f - __dn));
-    if (_FillValue)
+    if (_FillVal)
       *__first.__seg_ |= __m;
     else
       *__first.__seg_ &= ~__m;
@@ -194,13 +194,13 @@ __fill_n(__bit_iterator<_Cp, false> __first, typename _Cp::size_type __n) {
   }
   // do middle whole words
   __storage_type __nw = __n / __bits_per_word;
-  std::fill_n(std::__to_address(__first.__seg_), __nw, _FillValue ? static_cast<__storage_type>(-1) : 0);
+  std::fill_n(std::__to_address(__first.__seg_), __nw, _FillVal ? static_cast<__storage_type>(-1) : 0);
   __n -= __nw * __bits_per_word;
   // do last partial word
   if (__n > 0) {
     __first.__seg_ += __nw;
     __storage_type __m = ~__storage_type(0) >> (__bits_per_word - __n);
-    if (_FillValue)
+    if (_FillVal)
       *__first.__seg_ |= __m;
     else
       *__first.__seg_ &= ~__m;
@@ -1007,7 +1007,7 @@ private:
   friend class __bit_iterator<_Cp, true>;
   template <class _Dp>
   friend struct __bit_array;
-  template <bool _FillValue, class _Dp>
+  template <bool _FillVal, class _Dp>
   _LIBCPP_CONSTEXPR_SINCE_CXX20 friend void __fill_n(__bit_iterator<_Dp, false> __first, typename _Dp::size_type __n);
 
   template <class _Dp, bool _IC>