Cache memory buffer's name length
This avoids repeated calls to strlen while we already know its value.
When preprocessing sqlite3.c, this gives a surprising 2% speedup.
Full benchmark available here:
https://llvm-compile-time-tracker.com/compare.php?from=
d14c2d408dccd8c6defa7d151e9a96be3cac8cc3&to=
04f0641c1cbdcd0bdbd11cd910ca6091420bf52e&stat=instructions:u
Recommit
1824432174b3166b40bce59477beb5821170748e, with restored '\0' at the end of buffer name.
Differential Revision: https://reviews.llvm.org/D138555