[clang] [Clang] Implement Wpointer-bool-conversion-strict (PR #131523)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > I can't for the life of me see a good way to use this. The `if(p)` pattern is so common that no code base could ever turn on this warning. We could PERHAPS check that we're in the process of checking a condition expression and suppress this diagnostic, or recognize this pattern somehow, but without some more work to make this more useful, I don't think we can accept this diagnostic.
> 
> Given the amount of existing code out there which relies on pointer-to-bool conversions being well-defined, I don't think this makes sense as a compiler diagnostic. However, it could be a reasonable clang-tidy check as there are folks who find such code to be a type safety issue and they might want to have it diagnosed.

I find myself wondering if there is value/ability to just pattern match the operator= for a `bool` to see if it has said implicit/explicit casts on the RHS.  We unfortunately have to do this in something that handles the operator=, instead of the casts themselves (which are too ubiquitous to diagnose), but we should still be able to pattern-match this.

https://github.com/llvm/llvm-project/pull/131523