Remove -Wsigned-enum-bitfield from -Wmost. On a wide set of ABIs, this warning

Remove -Wsigned-enum-bitfield from -Wmost. On a wide set of ABIs, this warning
is completely irrelevant, producing (effectively) false positives, and -Wmost
is used pretty widely. We should somehow turn it back on by default when
targeting the MS ABI, however, since it indicates the program will not do as
intended in those cases.

(Or perhaps we should just treat enum bitfields as having the signedness of the
enum, even when targeting the MS ABI...)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@287193 91177308-0d34-0410-b5e6-96231b3b80d8