PPOs need to support drawing with Programs that failed their last
linkProgram() if they had previously successfully linked. This requires
saving the ProgramExecutable when linkProgram() succeeds, and not
overwriting it with subsequent linkProgram() calls unil the next
successful one.

To achieve this, the new member ProgramState::mLinkedExecutable will
point to the last successfully linked ProgramExecutable and
ProgramState::mExecutable will point to a new ProgramExecutable when the
next linkProgram() is attempted. If the link fails, the newly allocated
ProgramExecutable will be delete()'ed and mExecutable will point to the
previous 'good' ProgramExecutable still being tracked by
mLinkedExecutable. If it succeeds, the old mLinkedExecutable will be
delete()'ed and mLinkedExecutable will be updated to point to the ne
one.

Bug: angleproject:4514
Test: KHR-GLES31.core.sepshaderobjs.StateInteraction
Change-Id: I0677602a6d652a055404667ec9e9305fed5b4177
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2181450
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>