Added GL built-in functions to symbol table autogeneration
Built-in lookup now checks against spec and returns GL
symbols when appropriate.

Bug: angleproject:3719
Change-Id: I74786150f6063b40e13aebcb2eb9ea35e4ecaa53
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1731550Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Clemen Deng <clemendeng@google.com>