GetOperatorString is now used when writing GLSL for built-in calls
that fall under TIntermUnary. Component-wise not TOperator enum is
renamed for consistency.

This also cleans up some unnecessary creation of string objects when
writing built-in functions.

BUG=angleproject:1682
TEST=angle_unittests, angle_end2end_tests, WebGL conformance tests

Change-Id: I89b2ef222bf5af479d4977417f320789b58ace85
Reviewed-on: https://chromium-review.googlesource.com/424552
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>