GLSL, ESSL, and HLSL translators all had the same code to emulate
precision so migrate that code to base class.
This is a structural change to prepare for honoring precision
qualifiers in the Vulkan translator as well, which is done in a
follow-on commit.

Bug: angleproject:3078
Change-Id: Id85bba83543189e3bbc4002be92b056e63afc8de
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2072644Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>