[llvm] Update report_fatal_error calls from raw_string_ostream to use Twine(OS.str())

As described on D111049, we're trying to remove the <string> dependency from error handling and replace uses of report_fatal_error(const std::string&) with the Twine() variant which can be forward declared.

We can use the raw_string_ostream::str() method to perform the implicit flush() and return a reference to the std::string container that we can then wrap inside Twine().