[BOLT][NFC] Rename {MachO,}RewriteInstance::create methods

Follow the code style of fallible constructors in [LLVM Programmer's Manual]
(https://llvm.org/docs/ProgrammersManual.html#fallible-constructors)
and rename `RewriteInstance::createRewriteInstance` to `RewriteInstance::create`

Reviewed By: #bolt, rafauler

Differential Revision: https://reviews.llvm.org/D143119