Add the new structure 'UniformBlockIndex'. This is in preparation for
capturing glUniformBlockBinding() calls, which requires the (new)
corresponding ParamType::TUniformBlockIndex.

Bug: angleproject:5564
Change-Id: I88613dd96d058659dd253e8088fab49670c32316
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2651287
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>