cargo: Use the right LLVM tools and arguments for MSVC cross-compilation

`xbuild` uses the headers from `xwin` to support cross-compiling,
but doesn't fully replicate the other setup steps that it proposes to
provide a valid and working cross-compilation setup to MSVC.  We need to
make the following changes:

- `clang-cl` must be used (a `clang` driver with `cl.exe` interface),
  as all native projects will call `CC`/`CXX` with flags that are only
  compatible with `cl.exe`;
- `-I` does not seem to set up the system headers correctly, for this
  `-imsvc` should be used;
- `-fuse-ld=lld-link` does not strictly need to be set in `C(XX)FLAGS`
  unless a native project uses `CC`/`CXX` to link libraries
  (otherwise only `cargo`/`rustc` does this, but invokes it through the
   `LINKER` environment variable which is configured to `rust-lld`, as
   the default is otherwise `link.exe`).

xbuild/src/cargo/mod.rs

@@ -306,17 +306,18 @@ impl CargoBuild {
 
     pub fn use_windows_sdk(&mut self, path: &Path) -> Result<()> {
         let path = dunce::canonicalize(path)?;
-        self.cfg_tool(Tool::Cc, "clang");
-        self.cfg_tool(Tool::Cxx, "clang++");
+        self.cfg_tool(Tool::Cc, "clang-cl");
+        self.cfg_tool(Tool::Cxx, "clang-cl");
         self.cfg_tool(Tool::Ar, "llvm-lib");
-        self.cfg_tool(Tool::Linker, "rust-lld");
-        self.use_ld("lld-link");
+        self.cfg_tool(Tool::Linker, "rust-lld"); // Rust defaults to link.exe, use its rust-lld binary instead
+        self.use_ld("lld-link"); // Use lld's link.exe wrapper when a C(++) requests the linker to be used
+                                 // (Uncommon, typically all objects are only Ar-chived, and linked at once by cargo as the last build step)
         self.add_target_feature("+crt-static");
         self.add_cxxflag("-stdlib=libc++");
-        self.add_include_dir(&path.join("crt").join("include"));
-        self.add_include_dir(&path.join("sdk").join("include").join("um"));
-        self.add_include_dir(&path.join("sdk").join("include").join("ucrt"));
-        self.add_include_dir(&path.join("sdk").join("include").join("shared"));
+        self.add_msvc_include_dir(&path.join("crt").join("include"));
+        self.add_msvc_include_dir(&path.join("sdk").join("include").join("um"));
+        self.add_msvc_include_dir(&path.join("sdk").join("include").join("ucrt"));
+        self.add_msvc_include_dir(&path.join("sdk").join("include").join("shared"));
         self.add_lib_dir(&path.join("crt").join("lib").join("x86_64"));
         self.add_lib_dir(&path.join("sdk").join("lib").join("um").join("x86_64"));
         self.add_lib_dir(&path.join("sdk").join("lib").join("ucrt").join("x86_64"));
@@ -439,6 +440,10 @@ impl CargoBuild {
         self.c_flags.push_str(&format!("-I{} ", path.display()));
     }
 
+    pub fn add_msvc_include_dir(&mut self, path: &Path) {
+        self.c_flags.push_str(&format!("-imsvc{} ", path.display()));
+    }
+
     pub fn set_sysroot(&mut self, path: &Path) {
         let arg = format!("--sysroot={}", path.display());
         self.add_cflag(&arg);