Revert "Enable -Wsuggest-override in the LLVM build" and the follow-ups.

After lots of follow-up fixes, there are still problems, such as
-Wno-suggest-override getting passed to the Windows Resource Compiler
because it was added with add_definitions in the CMake file.

Rather than piling on another fix, let's revert so this can be re-landed
when there's a proper fix.

This reverts commit 21c0b4c1.
This reverts commit 81d68ad2.
This reverts commit a361aa52.
This reverts commit fa42b7cf.
This reverts commit 955f87f9.
This reverts commit 8b16e45f.
This reverts commit 308a127a.
This reverts commit 274b6b0c.
This reverts commit 1c7037a2.