Add libclang based C++ header validation

[dkorpel]

Fix regressions from #23176, as well as many other mismatches found by a WIP new CI check that I hope to push soon(tm)

Update: adds a new CI check that verifies that the manually maintained C++ headers in
include/dmd/ stay in sync with the D extern(C++) declarations in the compiler source.
Checks enum constant values, field offsets and sizes, class instance sizes, vtable indices, and C++ mangled names.

A python script uses clang to walk over all .h files in the include directory, and generate D asserts for enums, fields, and functions with C++ mangling. It currently generates:

https://gist.github.com/dkorpel/f8de8ca519e8dcfc2adcd691af6203e1

Then it runs it with the D compiler and -version=IN_LLVM (to skip fields in version (MARS) blocks). Added to the C++ test targets in build.d so you can run it with: ./build.d cpp-layout-test

[dlang-bot]

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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#23187"

[thewilsonator]

can we add to the tests to ensure that these specific changed do not regress in future?

[dkorpel]

I kinda meant to push this to another branch, but here we are. I pushed the new test target and updated the description.

[thewilsonator]

This is far more comprehensive than I meant to suggest. Nice!

[dkorpel]

I know, this is a response to Iain's rightful complaining about the frequently broken headers (#23176 (comment)). I tried fixing frontend.h for a bit to compile, but it's a real hassle, so I figured maybe it's simpler to generate D tests from C code rather than C tests (or headers) from D code.