[tint] Fix Objective-C files in Bazel

In https://dawn-review.googlesource.com/c/dawn/+/324475, it was
discovered objc files were not handled correctly and were temporarily
disabled.

This updates the generator to copy the .mm file to a .cc extension
and then stand-alone cc_library that compiles the objc file as
objc. By default, Bazel doesn't let .mm files go into cc_library
targets. It's a bit clunky but I couldn't get the official objc_library
to work on Tint's infrastructure (Something about the XCode setup
was not being found by Bazel's default apple rules).

While doing that, I noticed there were empty or effectively
empty select statements, so I cleaned those up by moving
the external dependency resolutions to go instead of being
in the template itself and adding helpers to see if a list
of files is non-empty before emitting the template section.

Finally, I noticed a missing header after
https://dawn-review.googlesource.com/c/dawn/+/324835 (at least on
GCC) and fixed it.

Suggested Review order:
 - src/tint/lang/msl/validate/BUILD.bazel to see the new objc->
   cc_library target and an omitted empty file list.
 - Other BUILD.bazel files to see more empty sections go away.
 - The go code (start with the tests)
 - Everything else.

Change-Id: I66360ebbf7fecdab1a95db00d3e20d4e6457e40e
Bug: b/256859233
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/325135
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>