[lldb] Enable C++14 when evaluating expressions in a C++14 frame

Summary:
Currently we never enable C++14 in the expression evaluator. This enables it when the language of the program is C++14.

It seems C++17 and so on isn't yet in any of the language enums (and the DWARF standard it seems), so C++17 support will be a follow up patch.

Reviewers: labath, JDevlieghere

Reviewed By: labath, JDevlieghere

Subscribers: aprantl

Differential Revision: https://reviews.llvm.org/D80308