Use INTERFACE_COMPILE_OPTIONS to disable -Wsuggest-override for any target that links to gtest

This cleans up several CMakeLists.txt's where -Wno-suggest-override was manually specified. These test targets now inherit this flag from the gtest target.

Some unittests CMakeLists.txt's, in particular Flang and LLDB, are not touched by this patch. Flang manually adds the gtest sources itself in some configurations, rather than linking to LLVM's gtest target, so this fix would be insufficient to cover those cases. Similarly, LLDB has subdirectories that manually add the gtest headers to their include path without linking to the gtest target, so those subdirectories still need -Wno-suggest-override to be manually specified to compile without warnings.

Differential Revision: https://reviews.llvm.org/D84554