Add docs for -foptimization-record-file=

[modocache]

Summary

The Clang option was previously not included in the User's Manual.

[hfinkel]

LGTM

[modocache]

Thanks for the review, Hal! :)

docs/UsersManual.rst

 **-fsave-optimization-record**
    Write optimization remarks to a YAML file.
 
    This option, which defaults to off, controls whether Clang writes
    optimization reports to a YAML file. By recording diagnostics in a file,
    using a structured YAML format, users can parse or sort the remarks in a
    convenient way.
 
+.. _opt_foptimization-record-file:
+
+**-foptimization-record-file**
+   Control the file to which optimization reports are written.
+
+   When optimization reports are being output (see
+   :ref:`-fsave-optimization-record <opt_fsave-optimization-record>`), this
+   option controls the file to which those reports are written.
+
+   If this option is not used, optimization records are output to a file named
+   after the primary file being compiled. If that's "foo.c", for example,
+   optimization records are output to "foo.opt.yaml".
+
 .. _opt_fdiagnostics-show-hotness:
 
 **-f[no-]diagnostics-show-hotness**
    Enable profile hotness information in diagnostic line.
 
    This option controls whether Clang prints the profile hotness associated
    with diagnostics in the presence of profile-guided optimization information.
    This is currently supported with optimization remarks (see