We the WARNINGS_AS_ERRORS CMake flag to treat all warnings as errors.

This change demotes the cherry-picked warnings-as-errors to just regular warnings, which can be optionally treated as errors like every other warning (WARNINGS_AS_ERRORS defaults to enabled).

Based on the discussions:
https://swiftshader-review.googlesource.com/c/SwiftShader/+/29810/6#message-de0432365b25c83372bac2f6528c79174aef0f6b
and
https://swiftshader-review.googlesource.com/c/SwiftShader/+/30191/2/CMakeLists.txt#248

Bug: b/135472272
Change-Id: I8aa37d79b9263560f9ff55ad4817f66ded6960ee
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32953
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>