ProgramPipeline's need to be able to link Programs before drawing, even
if the Program's previous linkProgram() failed. To work towards this,
some ProgramState members are being moved to ProgramExecutable so they
can be saved when linkProgram() succeeds and not overwritten by any
subsequent linkProgram() attempts that may fail.

This also allows the second half of this change, which is to pass in the
Program's ProgramExecutable to GlslangAssignLocations() and
GlslangWrapperVk::TransformSpirV() so the values saved from the last
successful linkProgram() are used.

Bug: angleproject:4514
Test: CQ
Change-Id: I68aa429be76c0c6e1b886be09a12200217fcc7ab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2181448
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>