[NFC][CLANG] Fix issue with dereference null return value found by Coverity static analyzer tool

[Manna]

Summary

Reported by Coverity:

In clang::​FunctionDecl::​isReplaceableGlobalAllocationFunction(std::​optional<unsigned int> *, bool *): Return value of function which returns null is dereferenced without checking 

if (!IsSizedDelete && !Ty.isNull() && Ty->isEnumeralType()) {
     QualType T = Ty;
     //Condition TD, taking false branch.
    while (const auto *TD = T->getAs<TypedefType>())
      T = TD->getDecl()->getUnderlyingType();
      //returned_null: getAs returns nullptr (checked 95 out of 97 times). 
    	
    //Dereference null return value (NULL_RETURNS)
    // dereference: Dereferencing a pointer that might be nullptr T->getAs() when calling getDecl. 
    IdentifierInfo *II = T->getAs<EnumType>()->getDecl()->getIdentifier();
    if (II && II->isStr("__hot_cold_t"))
      Consume();
  }

This patch uses castAs instead of getAs which will assert if the type doesn't match.

[Manna]

Thank you @erichkeane for review!

clang/lib/AST/Decl.cpp

bool FunctionDecl::isReplaceableGlobalAllocationFunction(
   // Finally, recognize the not yet standard versions of new that take a
   // hot/cold allocation hint (__hot_cold_t). These are currently supported by
   // tcmalloc (see
   // https://github.com/google/tcmalloc/blob/220043886d4e2efff7a5702d5172cb8065253664/tcmalloc/malloc_extension.h#L53).
   if (!IsSizedDelete && !Ty.isNull() && Ty->isEnumeralType()) {
     QualType T = Ty;
     while (const auto *TD = T->getAs<TypedefType>())
       T = TD->getDecl()->getUnderlyingType();
-    IdentifierInfo *II = T->getAs<EnumType>()->getDecl()->getIdentifier();
+    IdentifierInfo *II = T->castAs<EnumType>()->getDecl()->getIdentifier();
     if (II && II->isStr("__hot_cold_t"))
       Consume();
   }
 
   return Params == FPT->getNumParams();
 }
 
 bool FunctionDecl::isInlineBuiltinDeclaration() const {