Bundle clang+lld+libc++ for Windows

[Kha]

Building on top of #733. There are no official LLVM tarball releases for Windows and we need the mingw headers anyway, so we simply copy everything from mingw.

[Kha]

The size increase is more modest on Windows than on Linux: 110MB -> 150MB. This is presumably due to clang and lld sharing some of their code via libLLVM.dll, which is pretty much a necessity on Windows since you can't e.g. symlink lld-link to lld.

[Kha]

Uncompressed breakdown from CI:

lean-4.0.0-windows
    ├── [190M]  bin
    │   ├── [150K]  clang.exe
    │   ├── [1.7M]  lake.exe
    │   ├── [4.9M]  ld.lld.exe
    │   ├── [ 84K]  lean.exe
    │   ├── [107K]  leanc.exe
    │   ├── [ 860]  leanmake
    │   ├── [284K]  leanpkg.exe
    │   ├── [1.2M]  libc++.dll
    │   ├── [ 45M]  libclang-cpp.dll
    │   ├── [ 34K]  libffi-7.dll
    │   ├── [572K]  libgmp-10.dll
    │   ├── [1022K]  libiconv-2.dll
    │   ├── [ 44M]  libleanshared.dll
    │   ├── [ 90M]  libLLVM.dll
    │   ├── [162K]  liblzma-5.dll
    │   ├── [ 36K]  libunwind.dll
    │   ├── [1.5M]  libxml2-2.dll
    │   └── [104K]  zlib1.dll
    ├── [119K]  include
    │   ├── [ 40K]  clang
    │   │   ├── [ 857]  __stddef_max_align_t.h
    │   │   ├── [2.8K]  limits.h
    │   │   ├── [ 583]  stdalign.h
    │   │   ├── [1.1K]  stdarg.h
    │   │   ├── [7.1K]  stdatomic.h
    │   │   ├── [ 897]  stdbool.h
    │   │   ├── [3.5K]  stddef.h
    │   │   ├── [ 22K]  stdint.h
    │   │   ├── [ 510]  stdnoreturn.h
    │   │   └── [ 218]  windows.h
    │   └── [ 80K]  lean
    │       ├── [ 271]  config.h
    │       ├── [ 79K]  lean.h
    │       ├── [ 575]  lean_gmp.h
    │       └── [ 399]  version.h
    ├── [340M]  lib
    │   ├── [136K]  clang
    │   │   └── [136K]  12.0.1
    │   │       └── [136K]  lib
    │   │           └── [136K]  windows
    │   │               └── [136K]  libclang_rt.builtins-x86_64.a
    │   ├── [ 24K]  crt2.o
    │   ├── [ 396]  crtbegin.o
    │   ├── [ 396]  crtend.o
    │   ├── [ 17K]  dllcrt2.o
    │   ├── [333M]  lean
    │   │   ├── [ 24M]  Init ...
    │   │   ├── [7.4K]  Init.olean
    │   │   ├── [7.7M]  Lake ...
    │   │   ├── [8.9K]  Lake.olean
    │   │   ├── [220M]  Lean ...
    │   │   ├── [9.0K]  Lean.olean
    │   │   ├── [1.4M]  Leanpkg ...
    │   │   ├── [492K]  Leanpkg.olean
    │   │   ├── [3.2M]  libInit.a
    │   │   ├── [ 50M]  libLean.a
    │   │   ├── [4.4M]  libleancpp.a
    │   │   ├── [516K]  libleanrt.a
    │   │   ├── [ 12M]  libleanshared.dll.a
    │   │   ├── [990K]  libStd.a
    │   │   ├── [3.4M]  src ...
    │   ├── [146K]  libadvapi32.a
    │   ├── [ 12K]  libbcrypt.a
    │   ├── [1.9M]  libc++.a
    │   ├── [679K]  libc++.dll.a
    │   ├── [930K]  libgmp.a
    │   ├── [ 96K]  libgmp.dll.a
    │   ├── [435K]  libkernel32.a
    │   ├── [ 620]  libm.a
    │   ├── [108K]  libmingw32.a
    │   ├── [1.8M]  libmingwex.a
    │   ├── [ 628]  libmoldname.a
    │   ├── [540K]  libmsvcrt.a
    │   ├── [ 72K]  libpthread.a
    │   ├── [ 25K]  libpthread.dll.a
    │   ├── [ 69K]  libshell32.a
    │   ├── [ 47K]  libunwind.a
    │   ├── [6.2K]  libunwind.dll.a
    │   ├── [151K]  libuser32.a
    │   └── [   0]  windows
    └── [2.9K]  share
        └── [2.9K]  lean
            └── [2.9K]  lean.mk

 531M used in 142 directories, 1095 files

[tydeu]

@Kha why does the Windows distro bundle libLLVM but the Linux distro does not (at least per the listing in #733? That seems like something that should be consistent across platforms.

[Kha]

@tydeu I reported my guess on that above. But other than potential file size savings it really shouldn't matter. As I said in the Linux thread, this all should be regarded as implementation details.

[Kha]

Closing in favor of #795