Track expression language from one place in ClangExpressionParser
The current expression language is currently tracked in a few places within the ClangExpressionParser constructor.
This patch adds a private lldb::LanguageType attribute to the ClangExpressionParser class and tracks the expression language from that one place.
Author: Luke Drummond <email@example.com>
Differential Revision: http://reviews.llvm.org/D17719