[clang][driver] Enable MisExpect diagnostics flag outside of CC1

Previously we only accepted the `-fdiagnostics-misexpect-tolerance=` at
CC1, when it should have been handled identically to
`-fdiagnostics-hotness-threshold=`. It should not have been required to
pass this flag w/ `-Xclang` as reported here:
https://reviews.llvm.org/D115907#inline-1440745

Reviewed By: hans, phosek

Differential Revision: https://reviews.llvm.org/D149206