This CL will track whether a string has already been declared and
written in the output file. This cuts the shader string count in NBA2K20
trace by 85%, eliminating about 300K lines of code from setup.

Test: Compile and link NBA2K20 trace for Android
Bug: b/160014453
Bug: angleproject:4048
Change-Id: Id854b3314644c1e4aae41cd6bb4157f92e9b1945
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2339852Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>