[MC][BOLT] Add setter for AllowAtInName

Use the setter in BOLT to allow printing names with variant kind in the name
(e.g. "func@PLT").
Fixes BOLT buildbot tests that broke after D122516:
https://lab.llvm.org/buildbot/#/builders/215/builds/3595

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D122694