Minor fixes to for-loop warning.

The warning for unchanged loop variables outputted a diagnostic that was
dependent on iteration order from a pointer set, which is not always
deterministic.  Switch to a set vector, which allows fast querying and
preserves ordering.

Also make other minor changes in this area.
Use more range-based for-loops.
Remove limitation on SourceRanges that no logner exists.

llvm-svn: 304519