[cxx-interop] Move included stdlib headers from libstdcxx.h to the modulemap

[egorzhdan]

By referencing the C++ stdlib header directly from the modulemap, we make sure that the header is treated as a part of the stdlib, not a part of the first clang module to include it.

Addresses the issue described in https://forums.swift.org/t/llvm-fails-to-build-with-modules/59700/8.

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

@swift-ci please smoke test

[hyp]

This looks incorrect - Why do you want to do this only for those two headers? Shouldn't all the headers referenced in libstdcxx.h be in the module map?

[hyp]

You can also try making libstdcxx.h an umbrella header, since VFS places it into the libstdcxx directory.

[egorzhdan]

Yeap, you're right, we should try to refer to the stdlib headers in a consistent way. I moved these two headers only for now to make sure this doesn't cause too much fallout – moving all headers into the modulemap at once could produce a lot of compiler errors which would be hard to investigate.

I tried making libstdcxx.h an umbrella header in the past, the problem is that it causes clang to treat libstdc++ headers as different submodules, and that causes "duplicate definition" errors for types like size_t which are defined in multiple headers (behind something like #if SIZE_T_DEFINED).

[hyp]

Ok, in that case the header map should be generated dynamically in the compiler, and all the headers should be referenced in the module map to avoid issues like this. I don't see how this could cause additional fallout - at this point you're just playing whack a mole with the errors reported on the forums (there could be more), so wouldn't it make sense to address this correctly?

[egorzhdan]

A change like this could cause new modularization errors if, for example, there is a C++ stdlib header that is not directly referenced from the modulemap, but is #include-d from a user project. This happened when I tried moving tuple only (without functional) to the modulemap in this PR.

at this point you're just playing whack a mole with the errors reported on the forums (there could be more), so wouldn't it make sense to address this correctly?

I agree, I'm going to move more headers to the modulemap before merging this.

I wonder if it's possible to move the optional headers (those under #if __has_include) to the modulemap without having to generate the modulemap in the compiler. I guess it isn't, unless we inject empty headers into libstdc++ via VFS.

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

I will move the rest of the headers (those that are under #if __has_include) to the modulemap in a separate PR.