The mediump and lowp precisions translate to a RelaxedPrecision
decoration in SPIR-V.  This change implements this decoration and
enables SPIR-V generation when emulating Bresenham lines.  Prior to this
change, Bresenham line emulation resulted in a validation error due to
the vertex shader output being without RelaxedPrecision but the fragment
shader input (translated by glslang) with it.

The function that produces a new id is given a list of decorations to
apply, which itself is automatically deriven from the TType where
applicable, to minimize the risk of any id missing this decoration.

Bug: angleproject:4889
Change-Id: I30cc5a278634c5d83d63cd65b057336f9f573baa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2946113
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>