[Diagnostics] Improve recursion diagnostics, specifically for library shaders

[bob80905]

Previously, there would be no recursion validation performed on any function declarations in library shaders. This PR adds code that validates that no function declarations that are found inside library shaders are recursive. The diagnostics aren't repeated, since there's logic to prevent the same diagnostic from being emitted by 2 similar ancestors of the same recursive function. The PR also adjusts recursion detection for shaders in non-library cases, and improves the original diagnostic by adding information about the name of the function that is recursive. The PR is meant to complete the work on issue #5789. However, more work needs to be done for full accuracy.
The PR checks all function declarations in the translation unit for the library shader case, when it should really only be checking a subset of function declarations. This issue is filed here: #5857
Fixes #5789

[bob80905]

The new "CheckReachability" function is unused, but was helpful during iteration, and satisfies the description of the CallGraphWithRecurseGuard class. The class advertised that checking for reachability efficiently was one feature that was provided, but there isn't a function to do that explicitly, so I figured I'd add the function here.

[pow2clk]

Looks like the right direction! I think too much effort was made to keep the patch constant function special in the output. I think eliminating that requirement will simplify a lot.

[pow2clk]

Merp. I concur with the comments that Chris made that I've thumbed. I have some pushback on at least one, I'm not prepared to approve just yet if only because I think Chris had some good points.

Thanks for the responses to my earlier comments!

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[llvm-beanz]

I had one small comment, but otherwise this looks good.

[pow2clk]

I think some of Chris's comments still need addressing, but I'm satisfied with what's here

[pow2clk]

I just noticed that a lot of the end-to-end filecheck tests seem like they could be SemaHLSL verifier tests. Did you consider that?