C++ iterator types are often templated, and sometimes declare `operator==` as a non-member templated function. In libc++, an example of this is `__wrap_iter` which is used as an iterator type for `std::vector` and `std::string`.
We don't currently import templated non-member operators into Swift, however, we still want to support common C++ iterator patterns.
This change adds logic to instantiate templated non-member `operator==` for types that define `iterator_category` and are therefore likely to be valid iterator types.
rdar://97915515
C++ standard library module is called `std`. To make it more clear to a Swift developer that this module is a C++ stdlib, and not a Swift stdlib, let's rename it to `CxxStdlib`.
This is the first step in the module rename. We don't ban `std` in this patch to be able to build SwiftCompilerSources with hosttools until a new Swift compiler is shipped.
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
