Continue to emit notes for the candidates, but use different text.
Note that we can emit a typo correction fix-it even if there are
multiple candidates with the same name.
Also, disable typo correction in the migrator, since the operation
is quite expensive, the notes are never presented to the user, and
the fix-its can interfere with the migrator's own edits.
Our general guidance is that fix-its should be added on the main
diagnostic only when the fix-it is highly likely to be correct.
The exact threshold is debateable. Typo correction is certainly
capable of making mistakes, but most of its edits are right, and
when it's wrong it's usually obviously wrong. On balance, I think
this is the right thing to do. For what it's worth, it's also
what we do in Clang.
These variables really are local variables -- they have have a
TopLevelCodeDecl as their DeclContext rather than the SourceFile.
Treating them as global variables caused crashes in serialization.
We need an overhaul of top-level variables in script files anyway
(see rdar://problem/20992489&21628526), but this fixes the immediate
issue.
rdar://problem/21928533
Swift SVN r30519
