This patch changes function prototype handling so that they get assigned
the same kind of a mangled name as function definitions and function
calls. This name is now also printed in interm output so that function
prototypes can be accurately identified in the interm output.

BUG=angle:821
TEST=compiler_tests

Change-Id: Ia150b8ac5b816b8096c964767cd8666bdee28539
Reviewed-on: https://chromium-review.googlesource.com/227390Reviewed-by: Nicolas Capens <capn@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>