Sema: rename SemaBuiltinVAStart to SemaBuiltinVAStartMicrosoft

This function is used to perform semantic analysis on Microsoft style
`__va_start`.  Rename it to make this more explicit.  `__va_start` is
marked as `ALL_MS_LANGUAGES`, and requires Microsoft compatibility.
Other GNU targets will use `__builtin_va_start` instead.  NFC.

Addresses post-commit review comments from David Majnemer.

llvm-svn: 314241
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 962eb5d..f1d3131 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -833,7 +833,7 @@
     switch (Context.getTargetInfo().getTriple().getArch()) {
     case llvm::Triple::arm:
     case llvm::Triple::thumb:
-      if (SemaBuiltinVAStartARM(TheCall))
+      if (SemaBuiltinVAStartARMMicrosoft(TheCall))
         return ExprError();
       break;
     default:
@@ -3924,7 +3924,7 @@
   return false;
 }
 
-bool Sema::SemaBuiltinVAStartARM(CallExpr *Call) {
+bool Sema::SemaBuiltinVAStartARMMicrosoft(CallExpr *Call) {
   // void __va_start(va_list *ap, const char *named_addr, size_t slot_size,
   //                 const char *named_addr);