[NFC][CLANG] Fix static code analyzer concerns with dereference null return value

Reported by Static Code Analyzer Tool, Coverity:

Inside "SemaExprMember.cpp" file, in clang::Sema::BuildMemberReferenceExpr(clang::Expr *, clang::QualType, clang::SourceLocation, bool, clang::CXXScopeSpec &, clang::SourceLocation, clang::NamedDecl *, clang::DeclarationNameInfo const &, clang::TemplateArgumentListInfo const *, clang::Scope const *, clang::Sema::ActOnMemberAccessExtraArgs *): Return value of function which returns null is dereferenced without checking

  //Condition !Base, taking true branch.
  if (!Base) {
    TypoExpr *TE = nullptr;
    QualType RecordTy = BaseType;

     //Condition IsArrow, taking true branch.
     if (IsArrow) RecordTy = RecordTy->castAs<PointerType>()->getPointeeType();
    	//returned_null: getAs returns nullptr (checked 279 out of 294 times).
    	//Condition TemplateArgs != NULL, taking true branch.

     //Dereference null return value (NULL_RETURNS)
     //dereference: Dereferencing a pointer that might be nullptr RecordTy->getAs() when calling LookupMemberExprInRecord.
     if (LookupMemberExprInRecord(
           *this, R, nullptr, RecordTy->getAs<RecordType>(), OpLoc, IsArrow,
           SS, TemplateArgs != nullptr, TemplateKWLoc, TE))
        return ExprError();
     if (TE)
       return TE;

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

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D151130