Exposes a C API to name mangling for a given cursor.

Inspired by https://gist.github.com/tritao/2766291, and was previously discussed
on cfe-dev: http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-June/037577.html

Adding testing capability via c-index-test.




git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214410 91177308-0d34-0410-b5e6-96231b3b80d8