Reserved function names are now caught if the function is just
declared without being called in the shader source. Actually, function
calls don't need to be checked for reserved names, since that just
generates a redundant error message if function declarations are being
checked.

Includes some cleanup of ParseContext::checkIsNotReserved. It doesn't
need special handling of built-in symbols, as they are never passed to
the function.

BUG=chromium:739448
TEST=angle_unittests

Change-Id: I7115e1a7509626b5109b5c054c0704b0c3c19c58
Reviewed-on: https://chromium-review.googlesource.com/561457Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>