[Sema] Document+test the -Wsign-conversion change for enums in C code [NFC]

Basically a regression after r316268.
However the diagnostic is correct, but the test coverage is bad.

So just like rL316500, introduce yet more tests,
and adjust the release notes.

See https://bugs.llvm.org/show_bug.cgi?id=35200

llvm-svn: 317421