Instead of just validating definitions of main(), do the validation
for all function headers for functions named "main", including headers
in prototype declarations.

BUG=angleproject:1712
TEST=angle_unittests

Change-Id: Ia34a2a756e1cc27b241b27e8c01c6ef09bffba71
Reviewed-on: https://chromium-review.googlesource.com/430010Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>