Build LLVM for Windows using MSVC-compatible Clang (#497)

## Description

Switches the Windows LLVM build from Visual Studio to `clang-cl` +
`lld-link` (Clang's MSVC-compatible drivers) with Ninja.

This is in order to better match the Clang toolchain used on our Linux
and macOS variants, while keeping the MSVC ABI.

It also speeds up the build time by at least 2x on Windows.

crates/llvm-builder/src/platforms/x86_64_windows_msvc.rs

@@ -1,4 +1,8 @@
-//! The revive LLVM amd64 `windows-gnu` builder.
+//! The revive LLVM amd64 `windows-msvc` builder.
+//!
+//! Uses `clang-cl` and `lld-link` (Clang's MSVC-compatible drivers)
+//! to match the Clang toolchain used on our Linux and MacOS variants,
+//! while keeping the MSVC ABI and C++ runtime.
 
 use std::collections::HashSet;
 use std::process::Command;
@@ -27,6 +31,9 @@ pub fn build(
     sanitizer: Option<Sanitizer>,
 ) -> anyhow::Result<()> {
     crate::utils::check_presence("cmake")?;
+    crate::utils::check_presence("clang-cl")?;
+    crate::utils::check_presence("lld-link")?;
+    crate::utils::check_presence("ninja")?;
 
     let llvm_module_llvm =
         LLVMPath::llvm_module_llvm().and_then(crate::utils::path_windows_to_unix)?;
@@ -43,12 +50,16 @@ pub fn build(
                 "-B",
                 llvm_build_final.to_string_lossy().as_ref(),
                 "-G",
-                "Visual Studio 17 2022",
+                "Ninja",
                 format!(
                     "-DCMAKE_INSTALL_PREFIX='{}'",
                     llvm_target_final.to_string_lossy().as_ref(),
                 )
                 .as_str(),
+                format!("-DCMAKE_BUILD_TYPE='{build_type}'").as_str(),
+                "-DCMAKE_C_COMPILER='clang-cl'",
+                "-DCMAKE_CXX_COMPILER='clang-cl'",
+                "-DLLVM_USE_LINKER='lld-link'",
                 format!(
                     "-DLLVM_TARGETS_TO_BUILD='{}'",
                     targets
@@ -99,17 +110,7 @@ pub fn build(
         "LLVM building cmake",
     )?;
 
-    crate::utils::command(
-        Command::new("cmake").args([
-            "--build",
-            llvm_build_final.to_string_lossy().as_ref(),
-            "--target",
-            "install",
-            "--config",
-            build_type.to_string().as_str(),
-        ]),
-        "Building with msbuild",
-    )?;
+    crate::utils::ninja(llvm_build_final.as_ref())?;
 
     Ok(())
 }