Previously the modulemap for the C++ stdlib on Linux was provided via `-fmodule-map-file=` Clang argument pointing to the modulemap file within the Swift toolchain. The modulemap file could not reference the stdlib headers directly, since the exact stdlib include directory varies across Linux versions (it generally looks like `/usr/include/c++/{gcc_version}`). So the modulemap file instead referenced a local header, which `#include <>`-ed the stdlib headers, relying on Clang include resolution.
Unfortunately this did not work properly in the presence of another C++ module which included the stdlib headers: sometimes decls from the stdlib were hijacked by the other module, and were not treated as a part of the stdlib by Clang. This caused compile errors in Swift.
This change uses LLVM VFS to inject the modulemap file into the libstdc++ directory. The modulemap file is now able to reference the stdlib headers directly, which fixes the issue.
Credits to Rintaro Ishizaki for proposing a similar idea for SwiftGlibc back in 2016.
Since libstdc++ doesn't come with a Clang modulemap, Swift provides its own modulemap for libstdc++. This change makes the modulemap available while building SwiftCompilerSources.
This change adds a module map for libstdc++, which allows it to be properly imported into Swift as a module. The module map is installed into `usr/lib/swift/linux/{arch}` similarly to `glibc.modulemap`, and is passed to Clang as `-fmodule-map-file`.
That means it is now possible to import std directly from Swift on Linux, when C++ interop is enabled.
The module map currently declares a single `std` module without splitting the headers into submodules. This is going to change in the near future.
rdar://87654514
