[NFC] [clang] Use std::string instead of StringRef to reduce stack usage

Comparisons between StringRef and string literals are lowered by MSVC to happen on the stack.
All string literals are allocated unique stack slots increasing the overall stack usage of
calculateAttributeSpellingListIndex. Use of std::string forces allocation of a string type per
literal, reducing the overall stack usage of the function.

Change-Id: I7ac39fc96ab2e559318413fa26ef304c3fb0bd6e

clang/lib/Basic/Attributes.cpp

@@ -157,8 +157,10 @@ unsigned AttributeCommonInfo::calculateAttributeSpellingListIndex() const {
   // Both variables will be used in tablegen generated
   // attribute spell list index matching code.
   auto Syntax = static_cast<AttributeCommonInfo::Syntax>(getSyntax());
-  StringRef Scope = normalizeAttrScopeName(getScopeName(), Syntax);
-  StringRef Name = normalizeAttrName(getAttrName(), Scope, Syntax);
+  // We use std::string instead of StringRef to prevent local stack 
+  // allocation of literal strings for comparison.
+  const std::string Scope = normalizeAttrScopeName(getScopeName(), Syntax);
+  const std::string Name = normalizeAttrName(getAttrName(), Scope, Syntax);
 
 #include "clang/Sema/AttrSpellingListIndex.inc"
 }