Change raw_ostream so that it doesn't call llvm_report_error

immediately on every output error. Instead, add a flag to
raw_ostream, and set the flag whenever an error is detected.

The flag can be queried and cleared from the public API. This
gives applications more flexibility to handling errors in
application-specific ways.

If the flag is not cleared when the raw_ostream is destructed,
llvm_report_error is called from the destructor. This ensures
that errors are not implicitly silenced, and provides
convenient default behavior for tools like llc and opt.
Clients wishing to avoid llvm_report_error calls from
raw_ostream should check for errors and clear the error flag.

llvm-svn: 75857