Generate an error message when an user-defined function call is found in
a global variable initializer.

Even before this patch, the call graph already marked functions that were
only called from the global scope as unused.

This change was tested extensively with popular WebGL content, with no
regressions found.

TEST=angle_unittests
BUG=angleproject:988

Change-Id: Iec1b16d2af386f1e5c383f86926d80cef553b694
Reviewed-on: https://chromium-review.googlesource.com/283291Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>