Auto merge of #20895 - kwonoj:bump-dep, r=asajeffrey

build(servo): bump up llvm to 6.0.0

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This PR bumps up llvm to 6.0.0 to allow vs2017 build works on windows.

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [x] These changes fix #20888 (github issue number if applicable).

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python/servo/packages.py

@@ -4,7 +4,7 @@
 
 WINDOWS_MSVC = {
     "cmake": "3.7.2",
-    "llvm": "4.0.0",
+    "llvm": "6.0.0",
     "moztools": "0.0.1-5",
     "ninja": "1.7.1",
     "openssl": "1.1.0e-vs2015",