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.

.github/workflows/test-llvm-builder.yml

@@ -40,6 +40,18 @@ jobs:
         run: |
           brew install ninja
 
+      - name: Install Dependencies
+        if: matrix.runner == 'windows-2022'
+        run: |
+          choco install ninja
+
+      # Provides MSVC toolchain (ml64, headers, libs) used by clang-cl when building LLVM.
+      - name: Set Up MSVC Developer Environment
+        if: matrix.runner == 'windows-2022'
+        uses: ilammy/msvc-dev-cmd@v1
+        with:
+          arch: x64
+
       - name: Test
         run: make test-llvm-builder
         env: