This method doesn't really belong to GL, and was only used by the D3D
back-end to compute some attribute indexes. Simplify the code by
moving it into ProgramD3D. Also add the ability for the ShaderImpl to
assert that any pending compiles have resolved.

BUG=angleproject:1156

Change-Id: I0af3d3082ff8c908e6a87b9734989efbefd28808
Reviewed-on: https://chromium-review.googlesource.com/526336
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>