[NFC][CLANG] Fix nullptr dereference found by Coverity static analysis tool

Reported by Coverity:

  In clang::ASTContext::hasUniqueObjectRepresentations(clang::QualType, bool): Return value of function which returns null is dereferenced without checking.

  (Ty->isMemberPointerType()) {
      	//returned_null: getAs returns nullptr.
      	//var_assigned: Assigning: MPT = nullptr return value from getAs.
      const auto *MPT = Ty->getAs<MemberPointerType>();

     //dereference: Dereferencing a pointer that might be nullptr MPT when calling getMemberPointerInfo. (The virtual call resolves to
     <unnamed>::ItaniumCXXABI::getMemberPointerInfo.)
      return !ABI->getMemberPointerInfo(MPT).HasPadding;
    }

ABIs assume the parameter passed to `getMemberPointerInfo` is non-null.
This patch checks type by doing a `if (const auto *MPT = Ty->getAs<MemberPointerType>())` instead.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D149922