CentOS 7 comes with an outdated libstdc++ version. In that version, `std::basic_string` has a `mutable` member, which makes all of `basic_string`'s member functions mutating in Swift. This prevents us from calling `size()` or `empty()` on an immutable value of `std.string`.
rdar://91548568
rdar://91670704
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
* [cxx-interop] fix std::string::push_back by fixing mapping between clang/swift self/this type for cxx methods
* cleanup
* Fix unit test
* XFail test: operators/member-inline on linux-android
* add `C++` in expandExternalSignatureTypes comment
* Fix rebase
* Fix mapping issue in externalizeArguments
* Improve cxx_interop_ir test regex
