Checking against the max attribute count is not needed
as aliasing is implemented correctly below.  If not enough
locations are available, linking will fail.

This works for backends that support aliasing (Vulkan, GLES),
but not for D3D11, which asserts.  To handle this, add a
limitation that informs the frontend of aliased attribute support.

Bug: angleproject:3252
Bug: chromium:964404
Change-Id: Ib9ae5d381bbb2e6bf496272fc3d9e88467c17290
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1572817
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>