Add simple way for a CorrectionCandidateCallback to reject exact

matches of the typo.

Also be more proactive about checking a correction's visibility so that
a correction requiring a module import can be distinguished from the
original typo even if it looks identical. Otherwise the correction will
be excluded and the diagnostic about needing the module import won't be
emitted.

Note that no change was made to checkCorrectionVisibility other than
moving where it is at in SemaLookup.cpp.

llvm-svn: 220696