Make clang_Cursor_getMangling not mangle if the declaration isn't mangled

Right now clang_Cursor_getMangling will attempt to mangle any
declaration, even if the declaration isn't mangled (extern C).  This
results in a partially mangled name which isn't useful for much. This
patch makes clang_Cursor_getMangling return an empty string if the
declaration isn't mangled.

Patch by Michael Wu <mwu@mozilla.com>.

llvm-svn: 253909