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 <luke.drummond@codeplay.com>
Differential Revision: http://reviews.llvm.org/D17719

llvm-svn: 263099