[NFC][CLANG] Fix Static Code Analyzer Concerns

This patch uses castAs instead of getAs which will assert if the type doesn't match to resolve dereference issue with nullptr FPT when calling getThisType() in clang::CodeGen::CGDebugInfo::CreateType(clang::MemberPointerType const *, llvm::DIFile *).

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D151947