Move function header parsing code to ParseContext instead of having
it in the grammar file. This commit is pure refactoring. This code
needs to be changed later to detect returning arrays containing
structs.

This commit also disables clang format for glslang.cpp generated by
bison.

TEST=angle_unittests
BUG=angleproject:1015

Change-Id: If60848ca32da6b98ea8bcd95bba8c3f831634b51
Reviewed-on: https://chromium-review.googlesource.com/352480Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>