Incorrect casting in BitFlags Clear.

[bzbarsky-apple]

Doing foo &= bar does not do the right thing if foo and bar are both of a type that will get promoted to int when doing &. Unfortunately, that's what the code is doing after the modifications in #30680

This will happen to compile on some compilers/versions but not others.

Originally posted by @bzbarsky-apple in #30680 (comment)

[andy31415]

Do we happen to have a concrete compiler variant that exposes this? it would be nice to add this kind of test in CI when we fix it (and in case other places have similar issues that are compiler-dependent)

[bzbarsky-apple]

I don't yet; I would have mentioned it otherwise. It's just a landmine waiting to explode on someone for now.