Auto merge of #22756 - servo:jdm-patch-35, r=asajeffrey

Upgrade LLVM on windows.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #22636
- [x] There are tests for these changes

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appveyor.yml

@@ -1,5 +1,5 @@
 version: 1.0.{build}
-image: Previous Visual Studio 2017
+image: Visual Studio 2017
 
 environment:
   CCACHE_DIR: "%APPVEYOR_BUILD_FOLDER%\\.ccache"

python/servo/packages.py

@@ -4,7 +4,7 @@
 
 WINDOWS_MSVC = {
     "cmake": "3.7.2",
-    "llvm": "6.0.0",
+    "llvm": "7.0.0",
     "moztools": "3.2",
     "ninja": "1.7.1",
     "openssl": "1.1.0e-vs2015",