cmake: fix checks for compiler warning flags
cmake: fix checks for compiler warning flags

Summary:
gcc does not complain about unknown `-Wno-*` flags by default, and older
versions of gcc do not support `-Werror=unknown-warning-option`.  Therefore
these checks for `-Wno-noexcept-type` and `-Wno-nullability-completeness` could
incorrectly indicate that the compiler supports these flags.

This changes the checks to check against the positive version of the warning
flag.

Reviewed By: yfeldblum

Differential Revision: D7035995

fbshipit-source-id: b1511d31721aeb779859848e20820c8a7fea4faa

facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly