Add README with instructions for creating an LLVM bundle on Windows

CMakeSettings.json

@@ -0,0 +1,16 @@
+{
+  "configurations": [
+    {
+      "name": "x64-Clang-Release",
+      "generator": "Ninja",
+      "configurationType": "Release",
+      "buildRoot": "${projectDir}\\out\\build\\${name}",
+      "installRoot": "${projectDir}\\out\\install\\${name}",
+      "cmakeCommandArgs": "-DLLVM_ENABLE_PROJECTS=clang",
+      "buildCommandArgs": "",
+      "ctestCommandArgs": "",
+      "inheritEnvironments": [ "clang_cl_x64_x64" ],
+      "variables": []
+    }
+  ]
+}

README.md

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+# Custom LLVM bundles #
+
+This is a repository to hold custom built LLVM bundles.
+
+## Windows ##
+
+* Download the desired version of LLVM.
+
+* Place the file `CMakeSettings.json` that is in this repository within the directory called `llvm` within the downloaded LLVM.
+
+* Open the directory `llvm` with Visual Studio and let it configure with the `CMakeSettings.json`.
+
+* Build and install using the `x64-Clang-Release` configuration.
+
+* The install should be in the `out\install` directory, bundle it using `cmake` as:
+
+```sh
+$ cd .\out\install\
+# NOTE: Switch out the version (here 11.0.0)
+#       This takes a long time
+$ cmake -E tar -cJ clang+llvm-11.0.0-x86_64-windows.tar.xz x64-Clang-Release
+```