[lldb][NFC] Remove GetASTContext call in ClangPersistentVariables

We try to build a CompilerType from the persistent decls so we need
a ClangASTContext. With this patch the ClangPersistentVariables store
the associated ClangASTContext of the persistent decls (which is
always the scratch ClangASTContext) and no longer call GetASTContext
to map back from clang::ASTContext to ClangASTContext.