[clang] NFC: Use flush() idiomatically

Using both `raw_ostream::flush()` and `raw_ostream::str()` consecutively is
redundant. The alternatives are:

- Use `raw_ostream::str()` without `raw_ostream::flush()`
- Use `raw_ostream::flush()` and then use the destination for `raw_ostream`
  writer

The latter is more idiomatic, so the fix resolves this particular case in its
favor.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D118247