Fix VM llvm-project submodule contamination (#1605)

Co-authored-by: Curtis J Bezault <curtbezault@gmail.com>

azure-devops/checkout-sources.yml

@@ -43,9 +43,10 @@ steps:
     )
 
     git fetch --filter=tree:0 --depth=1 llvm $(${{ parameters.llvmSHAVar }})
-    git reset --quiet FETCH_HEAD
     git sparse-checkout init --cone
     git sparse-checkout set libcxx/test libcxx/utils/libcxx llvm/utils/lit
+    git reset --quiet --hard FETCH_HEAD
+    git clean --quiet -x -d -f
   displayName: "Checkout LLVM source"
 - script: |
     cd $(Build.SourcesDirectory)