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

[Manna]

Summary

Reported by Coverity static analyzer tool:

Inside "ItaniumCXXABI.cpp" file, in <unnamed>::​ItaniumCXXABI::​EmitLoadOfMemberFunctionPointer(clang::​CodeGen::​CodeGenFunction &, clang::​Expr const *, clang::​CodeGen::​Address, llvm::​Value *&, llvm::​Value *, clang::​MemberPointerType const *): Return value of function which returns null is dereferenced without checking.

 //returned_null: getAs returns nullptr (checked 130 out of 156 times). 
 //var_assigned: Assigning: FPT = nullptr return value from getAs.
 const FunctionProtoType *FPT =
   MPT->getPointeeType()->getAs<FunctionProtoType>();
auto *RD =
   cast<CXXRecordDecl>(MPT->getClass()->castAs<RecordType>()->getDecl());

  	
// Dereference null return value (NULL_RETURNS)
//dereference: Dereferencing a pointer that might be nullptr FPT when calling arrangeCXXMethodType.
 llvm::FunctionType *FTy = CGM.getTypes().GetFunctionType(
     CGM.getTypes().arrangeCXXMethodType(RD, FPT, /*FD=*/nullptr));

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

[Manna]

Thank you @erichkeane for reviews!

[Manna]

Fix Clang-format errors

clang/lib/CodeGen/ItaniumCXXABI.cpp

 /// the function to call.
 CGCallee ItaniumCXXABI::EmitLoadOfMemberFunctionPointer(
     CodeGenFunction &CGF, const Expr *E, Address ThisAddr,
     llvm::Value *&ThisPtrForCall,
     llvm::Value *MemFnPtr, const MemberPointerType *MPT) {
   CGBuilderTy &Builder = CGF.Builder;
 
   const FunctionProtoType *FPT =
-    MPT->getPointeeType()->getAs<FunctionProtoType>();
+      MPT->getPointeeType()->castAs<FunctionProtoType>();
   auto *RD =
       cast<CXXRecordDecl>(MPT->getClass()->castAs<RecordType>()->getDecl());
 
   llvm::FunctionType *FTy = CGM.getTypes().GetFunctionType(
       CGM.getTypes().arrangeCXXMethodType(RD, FPT, /*FD=*/nullptr));
 
   llvm::Constant *ptrdiff_1 = llvm::ConstantInt::get(CGM.PtrDiffTy, 1);