As glslang is rolling frequently now, internal shaders frequently
require recompilation.  This change parallelizes shader compilation to
speed up this process.

Bug: angleproject:3333
Change-Id: Icace083559bff73dfb9b5fe7cc2c59ce8137a2dc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1558680
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>