Move GetPlatformInfo implementation from stubs to cl::Platform,
because stubs are meant to be small.
Also move CLPlatformImpl::Info instance to cl::Platform as it is
more efficient to be accessed from there, and more consistent with
cl::Device.

Bug: angleproject:5904
Change-Id: I4c0ba6390467d6ef357e38a5ef08f54b0a88d423
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2880663
Commit-Queue: John Plate <jplate@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>