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=5279e6a7d677cdf4488883b77aacab911318100c&to=389601b0dbdf23cf25167ddfc49b3af5742ebd9a&stat=instructions:u

Differential Revision: https://reviews.llvm.org/D138555