Update LLVM libraries to v19.1

[sbc100]

It looks like LLVM 19.1 was released back in september but all our core libraries are still based on LLVM 18.

[sbc100]

@aheejin do you fancy doing this again? If not I'm happy to take this one.

[aheejin]

Sure!

[ShuangLiu1992]

libcxx 19 incldues generates a std.cppm which can be used for compiling a C++ 20 modules interface, would it be possible to include them so that they can be consumed? CMake supports import std; for Clang + libcxx + ninja now.
https://www.kitware.com/import-std-in-cmake-3-30/

[sbc100]

While C++ module support is certainly something we want to support, I don't want to block this update on that. We can add C++ 20 module support (along with tests) as a followup I think.

[ShuangLiu1992]

Would it be possible to simply install the std.cppm and libc++.modules.json along the emscripten installation without testing? I have managed to compile the latest libcxx with Emscripten and get these files.

And along with a script that strips certain emcc specific flags and passing it clang-scan-deps and I have managed to use module successfully with import std; and on moderately sized projects with handwritten module wrappers for functions and struct of interest (Eigen, OpenCV and Boost etc).

Understand that proper testing is going to take longer and more work but it should be OK to at least include these files as they are simply part of libcxx installation in 19.1.4?

[sbc100]

Understand that proper testing is going to take longer and more work but it should be OK to at least include these files as they are simply part of libcxx installation in 19.1.4?

Sure, if they are simply part of libcxx then we can include them. If they need to be built some source that might be harder since out "import" process for libcxx is just copying stuff out of a git checkout.. there is no build step.

If a build step is required then likely modules will be a followup PR. In any case its certainly some that can and should work ASAP.

[sbc100]

See #22305 for more on C++20 modules.

[aheejin]

#22994 currently does not contain modules/ directory. I'll make that clear in the PR description later. It can be added later as a follow-up.

[aheejin]

Closed by #22934, #22937, and #22994.