[clang] Support building native tools when cross-compiling standalone clang

When cross-compiling the LLVM project as a whole (from llvm/), if it
cannot find presupplied tools it will create a native build environment
to build the tools it needs.

However, when doing a standalone build of clang (that is, from clang/
and linking against an existing libLLVM) this doesn't work. Instead
a _target_ binary is built which predictably then fails.

The conventional workaround for this is to build the native tools in a
separate native compile phase and pass the paths to the cross build, for
example see OpenEmbedded[1] or Nix[2]. But we can do better!

The first problem is that LLVM_USE_HOST_TOOLS is only set in the llvm/
CMakeLists.txt, so setup_host_tool() will never consider building a
native binary.  This can be solved by setting LLVM_USE_HOST_TOOLS based
on CMAKE_CROSSCOMPILING in clang/CMakeLists.txt in the standalone case.

Now setup_host_tool() will try to build a native tool, but it needs
build_native_tool() from CrossCompile.cmake, so that also needs to be
included.

Finally, the native binary then fails because there's no provider for
the dependency "CONFIGURE_Clang_NATIVE", so use llvm_create_cross_target
to create the native environment.

These few lines mirror what the lldb CMakeLists.txt does in the
standalone case, so there is prior art for this.

[1] https://git.openembedded.org/openembedded-core/tree/meta/recipes-devtools/clang/clang_git.bb?id=e18d697e92b55e57124e80234369d46575226386#n212
[2] https://github.com/NixOS/nixpkgs/blob/3354d448f2a26117a74638957b0131ce3da9c8c4/pkgs/development/compilers/llvm/common/tblgen.nix#L54

Author: rossburton

clang/CMakeLists.txt

@@ -80,6 +80,12 @@ if(CLANG_BUILT_STANDALONE)
   include(GetErrcMessages)
   include(LLVMDistributionSupport)
 
+  if(CMAKE_CROSSCOMPILING)
+    set(LLVM_USE_HOST_TOOLS ON)
+    include(CrossCompile)
+    llvm_create_cross_target(Clang NATIVE "" Release)
+  endif()
+
   set(PACKAGE_VERSION "${LLVM_PACKAGE_VERSION}")
   set(BUG_REPORT_URL "${LLVM_PACKAGE_BUGREPORT}" CACHE STRING
     "Default URL where bug reports are to be submitted.")