ci: build LLVM with thread support on Windows

This should fix a number of concurrency/threading issues.

I had to force-disable concurrency in the linker using a hack. I'm not
entirely sure what the cause is, possibly the MinGW version (version 12
appears to work for me, while version 11 as used on the GitHub runner
image seems to be broken).
There are a few ways to fix this in a better way:
  * Fix the underlying cause (possibly by upgrading to MinGW-w64 12).
  * Add the `--threads` flag to the LLD MinGW linker, so we can use a
    regular parameter instead of this hack.

.github/workflows/windows.yml

@@ -66,7 +66,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-15-windows-v5
+          key: llvm-build-15-windows-v6
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'