Further update -Wbitfield-constant-conversion for 1-bit bitfield

https://reviews.llvm.org/D131255 (82afc9b1)
began warning about conversion causing data loss for a single-bit
bit-field. However, after landing the changes, there were reports about
significant false positives from some code bases.

This alters the approach taken in that patch by introducing a new
warning group (-Wsingle-bit-bitfield-constant-conversion) which is
grouped under -Wbitfield-constant-conversion to allow users to
selectively disable the single-bit warning without losing the other
constant conversion warnings.

Differential Revision: https://reviews.llvm.org/D132851