[clang] Don't modify OptRemark if the argument is not relevant

A followup to D110201.

 For example, we'd set OptimizationRemarkMissed's Regex to '.*' when
encountering -Rpass. Normally this doesn't actually affect remarks we
emit because in clang::ProcessWarningOptions() we'll separately look at
all -R arguments and turn on/off corresponding diagnostic groups.
However, this is reproducible with -round-trip-args.

Reviewed By: JamesNagurne

Differential Revision: https://reviews.llvm.org/D110673