[clang][MinGW] Add `-mguard=cf` and `-mguard=cf-nochecks`

This option can be used to enable Control Flow Guard checks and
generation of address-taken function table. They are equivalent to
`/guard:cf` and `/guard:cf,nochecks` in clang-cl. Passing this flag to
the Clang driver will also pass `--guard-cf` to the MinGW linker.

This feature is disabled by default. The option `-mguard=none` is also
available to explicitly disable this feature.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D132810