Rename Mangled::GetLanguage to Mangled::GuessLanguage

The language can not be definitively determined from the mangling, so
this new name helps clarify that fact.  This addresses the concerns raised
in http://reviews.llvm.org/rL226962.
Reviewed by: clayborg
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D10723

llvm-svn: 240662