Store OptTable::Info::Name as a StringRef

This avoids implicit conversion to StringRef at several points, which in
turns avoid redundant calls to strlen.

As a side effect, this greatly simplifies the implementation of
StrCmpOptionNameIgnoreCase.

It also eventually gives a consistent, humble speedup in compilation
time.

https://llvm-compile-time-tracker.com/compare.php?from=5f5b942823474e98e43a27d515a87ce140396c53&to=60e13b778119fc32d50dc38ff1a564a87146e9c6&stat=instructions:u

Differential Revision: https://reviews.llvm.org/D139274