[lldb][NFC] Refactor some IsClangType checks in ClangASTContext

Summary:
All type in these functions need be valid and Clang types, so
we might as well replace these checks with IsClangType.

Also lets IsClangType explicitly check for validity instead of
assuming that the TypeSystem is a nullptr.

Subscribers: abidh, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70001