[NFC][CLANG] [NFC][CLANG] Fix nullptr dereference issue in ConvertQualTypeToKind()

This patch uses castAs instead of getAs which will assert if the type doesn't match in ConvertQualTypeToKind(clang::ASTContext const &, clang::QualType).

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D151928