When attempting to link a new program, the program must be marked
as unlinked, but the validation-related caching should not be
cleared yet, since we can still use the previously linked program
if linking fails at this point.

Added an angle end2end test which fails without this fix and
passes with this fix.

Fixes the following WebGL 1.0.4 test with SwANGLE:
conformance/programs/program-test.html

Bug: angleproject:3557
Change-Id: Ib6722ba88803979e8f292c9b7b81f85cc0304662
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2173538Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>