[Expr] Check the language before ignoring Objective C keywords
This patch adds the check of the language before ignoring names like id or
Class, which are reserved in Objective C, but are allowed in C++. It is needed
to make it possible to evaluate expressions in a C++ program containing names
like id or Class.
Reviewers: jingham, zturner, labath, clayborg
Reviewed By: jingham, clayborg
Differential Revision: https://reviews.llvm.org/D54843