Fix crash when an 'import a module' TypoCorrection has its CorrectionDecls

replaced by visible decls.

Make sure that all paths through checkCorrectionVisibility set the
RequiresImport flag appropriately, so we don't end up using a stale value.
Patch by Jorge Gorbe!

Differential Revision: https://reviews.llvm.org/D30963

llvm-svn: 304745