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

rL316268 / D39122 has fixed PR35009, and now when in C,
these three(?) diagnostics properly use the enum's underlying
datatype.

While it was fixed, the test coverage was clearly insufficient,
because the -Wsign-compare change didn't show up in any of the
tests, until it was reported in the post-commit mail for rL316268.

So add the test for the -Wsign-compare diagnostic for enum
for C code, and while there, document this in the release notes.

The fix itself was obviously correct, so unless we want to silence
this new diagnosed case, i deem this commit to be NFC.

llvm-svn: 316500