[libcxx] Enable filesystem by default for mingw targets

[mstorsjo]

Summary

This feature can be built successfully for windows now, and should be fairly complete. However, the helper functions for __int128_t aren't available in MSVC configurations, so don't enable it by default there yet. (See https://reviews.llvm.org/D91139 for discussion on how to proceed with things in MSVC environments.)

[curdeius]

LGTM. Would it be good time soon too add some documentation about testing on Windows, now that other patches has landed?

[mstorsjo]

In D97075#2575315, @curdeius wrote:

LGTM. Would it be good time soon too add some documentation about testing on Windows, now that other patches has landed?

Yep, I'll pick up working on that now, plus refreshing the patches for the tests, and a few other things I've promised to post later, i.e. now.

libcxx/CMakeLists.txt

 include(HandleCompilerRT)
 
 # Basic options ---------------------------------------------------------------
 option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." OFF)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
 option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
 set(ENABLE_FILESYSTEM_DEFAULT ON)
-if (WIN32)
+if (WIN32 AND NOT MINGW)
+  # Filesystem is buildable for windows, but it requires __int128 helper
+  # functions, that currently are provided by libgcc or compiler_rt builtins.
+  # These are available in MinGW environments, but not currently in MSVC
+  # environments.
   set(ENABLE_FILESYSTEM_DEFAULT OFF)
 endif()
 option(LIBCXX_ENABLE_FILESYSTEM "Build filesystem as part of the main libc++ library"
     ${ENABLE_FILESYSTEM_DEFAULT})
 option(LIBCXX_INCLUDE_TESTS "Build the libc++ tests." ${LLVM_INCLUDE_TESTS})
 option(LIBCXX_ENABLE_PARALLEL_ALGORITHMS "Enable the parallel algorithms library. This requires the PSTL to be available." OFF)
 option(LIBCXX_ENABLE_DEBUG_MODE_SUPPORT
   "Whether to include support for libc++'s debugging mode in the library.