ArchiveFileDescriptorTests: Resurrect part of r197600, but make it invalidated, to appease buildbots.

Please revert this several hours later ;)
ArchiveFileDescriptorTests: Fix CMake build. Each unit test is expected to have suffix "*Tests" for lit gtest runner to seek one.
Added a comment about the launch_bounds attribute's AST node being required. Since there were no test cases for the attribute, some have been added. This promptly demonstrated a bug with the semantic handling, which is also fixed.