[Expr] Check the language before ignoring Objective C keywords

Summary:
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

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D54843

llvm-svn: 348240