[clang-tidy] Ignore bool -> single bit bitfield conversion in readability-implicit-bool-conversion

Summary: There is no ambiguity / information loss in this conversion

Reviewers: alexfh, aaron.ballman, hokein

Reviewed By: alexfh

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D54941

llvm-svn: 347671