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
Roman Lebedev