Fix regression reported by Michael Esser and Andrew Watson

(independently).

In our recently switch to llvm::raw_ostream (and friends) (I think this
is d934d983692c8952cdb887cbcd59f2df0001b9c0) we forgot to flush the
llvm::raw_string_ostream to the underlying string used for error report
files (e.g. test000001.overshift.err) so we would end up writing an
empty string to error report files.

Also added a test case to catch this.