Check for lack of C++ context first when demangling

Summary: It seems that if we have no context, then it can't possibly be a method.  Check that first.

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D32708
Patch by Scott Smith <scott.smith@purestorage.com>.

llvm-svn: 302008