[libc++][ABI Break] Make is_error_condition_enum_v and is_error_code_enum_v bool, not size_t

`is_error_condition_enum_v` and `is_error_code_enum_v` are currently of
type `size_t`, but the standard mandates they are of type `bool`.

This is an ABI break technically since the size of these variable
templates has changed. Document it as such in the release notes.

Fixes https://bugs.llvm.org/show_bug.cgi?id=50755

Reviewed By: ldionne, Quuxplusone, #libc, var-const

Differential Revision: https://reviews.llvm.org/D112553