Enables the translator to handle non-constant global initializers in
all essl versions iff the shader enables the extension to do so.

Bug: angleproject:4468
Test: angle_end2end_tests --gtest_filter=ShaderNonConstGlobalInitializerTest.*
Change-Id: I8f138c12fc83d2f38ff8f45ca9133222b01e4087
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2102959Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>