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

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 21c0b4c1e8d6a171899b31d072a47dac27258fc5.
This reverts commit 81d68ad27b29b1e6bc93807c6e42b14e9a77eade.
This reverts commit a361aa5249856e333a373df90947dabf34cd6aab.
This reverts commit fa42b7cf2949802ff0b8a63a2e111a2a68711067.
This reverts commit 955f87f947fda3072a69b0b00ca83c1f6a0566f6.
This reverts commit 8b16e45f66e24e4c10e2cea1b70d2b85a7ce64d5.
This reverts commit 308a127a38d1111f3940420b98ff45fc1c17715f.
This reverts commit 274b6b0c7a8b584662595762eaeff57d61c6807f.
This reverts commit 1c7037a2a5576d0bb083db10ad947a8308e61f65.