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

1 files changed, 7 insertions, 0 deletions

diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 2e989f0b..19d8692 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/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);