Store OptTable::Info::Name as a StringRef

This is a recommit of 8ae18303,
with a few cleanups.

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 (timing updated since original commit).

https://llvm-compile-time-tracker.com/compare.php?from=76fcfea283472a80356d87c89270b0e2d106b54c&to=b70eb1f347f22fe4d2977360c4ed701eabc43994&stat=instructions:u

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