[Symbol] Improve Variable::GetLanguage

Summary:
When trying to ascertain what language a variable belongs to, just
checking the compilation unit is often not enough. In r364845 I added a way to
check for a variable's language type, but didn't put it in Variable itself.
Let's go ahead and put it in Variable.

Reviewers: jingham, clayborg

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D64042

llvm-svn: 366733