Currently still ignoring precision qualifiers for Vulkan shaders by
default, but have added feature "enablePrecisionQualifiers" that can be
enabled in order to include precision qualifiers.

With this initial implementation, it's possible to get precision
qualifier mis-matches in the generated GLSL 4.50. According to the
spec this is allowed. From GLSLangSpec 4.50 section 4.7 "Precision and
Precision Qualifiers":

For the purposes of determining if an output from one shader stage
matches an input of the next stage, the precision qualifier need not
match.

However, when converted to SPIR-V and run through the shader validation
any mismatches will cause shader validation errors. Initially just
ignoring those errors with this commit.

Bug: angleproject:3078
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057749Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>
Change-Id: Ieecca604bb2c834c9b1c2bcab85279d1f8755dfa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2086280