Share code for finding the main function from the AST between
InitializeVariables, DeferGlobalInitializers,
EmulateGLFragColorBroadcast and UseInterfaceBlockFields. This makes
InitializeVariables simpler in particular, as it doesn't need an AST
traverser anymore.

BUG=angleproject:2033
TEST=angle_unittests, WebGL conformance tests

Change-Id: I14c994bbde58a904f6684d2f0b72bd8004f70902
Reviewed-on: https://chromium-review.googlesource.com/501166Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>