Forbid implicit conversion of constraint expression to bool

As reported in https://github.com/llvm/llvm-project/issues/54524, and
later in https://github.com/llvm/llvm-project/issues/60038, we were not
properly implmenting temp.constr.atomic P3. This patch stops implicitly
converting constraints to bool, and ensures the Rvalue conversion takes
place as needed.

Differential Revision: https://reviews.llvm.org/D141954