Revert "[Modules] Fix using va_list with modules and a precompiled header." (#101752)

Reverts #100837

The test Modules/builtin-vararg.c is failing on AArch64 build bots:
- https://lab.llvm.org/buildbot/#/builders/190/builds/3117
- https://lab.llvm.org/buildbot/#/builders/65/builds/2302
- https://lab.llvm.org/buildbot/#/builders/154/builds/2288

Revert to get the bots back to green.

Author: dyung

clang/lib/Sema/Sema.cpp

@@ -310,13 +310,6 @@ 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);