[cxx-interop] Allow compiling with libc++ on Linux

[egorzhdan]

This makes sure that Swift respects -Xcc -stdlib=libc++ flags.

Clang already has existing logic to discover the system-wide libc++ installation on Linux. We rely on that logic here.

Importing a Swift module that was built with a different C++ stdlib is not supported and emits an error.

The Cxx module can be imported when compiling with any C++ stdlib. The synthesized conformances, e.g. to CxxRandomAccessCollection also work. However, CxxStdlib currently cannot be imported when compiling with libc++, since on Linux it refers to symbols from libstdc++ which have different mangled names in libc++.

rdar://118357548 / #69825

[compnerd]

I'm not sure that this is great - the platform default c++ runtime could be libc++. I'd rather that we use a string parameter and let the frontend specify the runtime name.

[al45tair]

+1. I think this only works if the options are platform default versus libc++, but the latter is the platform default in many cases, and on Linux in general I think it'd make sense to allow libstdc++ on distros where the default happens to be libc++.

[egorzhdan]

I changed this to explicitly list the supported C++ stdlibs by name.

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

@swift-ci please smoke test

[al45tair]

I think we should be using more than one bit in the swiftmodule file, and that we should identify which C++ library the module was built with (rather than "default" versus "overridden", which is what we have now).

[al45tair]

+1. I think this only works if the options are platform default versus libc++, but the latter is the platform default in many cases, and on Linux in general I think it'd make sense to allow libstdc++ on distros where the default happens to be libc++.

[al45tair]

Suggested change

	enum class CXXStdlibKind : uint8_t {
	/// Use the default C++ stdlib for a particular platform: libc++ for Darwin,
	/// libstdc++ for most Linux distros.
	PlatformDefault = 0,
	/// Use libc++ instead of the default C++ stdlib on a platform where libc++
	/// is not the default, e.g. Ubuntu.
	OverrideLibcxx = 1,
	};
	enum class CXXStdlibKind : uint8_t {
	/// Use the default C++ stdlib for a particular platform: libc++ for Darwin,
	/// libstdc++ for most Linux distros. This should never be stored in a
	/// .swiftmodule file; we should always use one of the other constants
	/// to indicate exactly what was used at build time.
	PlatformDefault = 0,
	Libcxx = 1,
	Libstdcxx = 2,
	Msvcprt = 3,
	// Other C++ libraries may be supported in future
	};

[egorzhdan]

@swift-ci please smoke test

[hyp]

I tested this on windows, with some additional changes for custom libc++ and split out MSVC c++ support using sealing like before, and this kind of approach works great. This looks good to me.

[egorzhdan]

Thanks @hyp for trying it out!

[egorzhdan]

@swift-ci please smoke test

[compnerd]

Thank you for all the iteration. This really seems significantly better than both the first revision and even some of the original code.

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

@swift-ci please build toolchain UBI9