Allow only constants, other globals, temporaries, and uniforms to be used
in global variable initialization for now. The spec limits global
variable initialization further to only constant expressions, but fully
implementing this has a too large compatibility cost for ESSL 1.00, so
implement it only partially. In the case of ESSL 3.00 we can use stricter
validation, since there's no legacy to worry about.

TEST=angle_unittests, WebGL conformance tests
BUG=angleproject:988

Change-Id: I6a66f6a31130b44717dd2bef3082a0fc395a60b6
Reviewed-on: https://chromium-review.googlesource.com/270430Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>