[llvm] [docker][Windows] install libxml2 using vcpkg (PR #209422)

[Aiden Grossman via llvm-commits]

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@@ -48,6 +48,15 @@ RUN choco install -y ninja --version 1.13.1 && \
     choco install -y python3 --version 3.12.3 && \
     choco install -y make --version 4.4.1
 
+# Install libxml2 using vcpkg.
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boomanaiden154 wrote:

Add a note here why we are using `vcpkg`.

https://github.com/llvm/llvm-project/pull/209422