Attempt to upgrade compiler used by appveyor builds

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357632 91177308-0d34-0410-b5e6-96231b3b80d8

appveyor-reqs-install.cmd

@@ -9,7 +9,7 @@ cd C:\projects\deps
 :: Setup Compiler
 ::###########################################################################
 if NOT EXIST llvm-installer.exe (
-  appveyor DownloadFile https://prereleases.llvm.org/win-snapshots/LLVM-8.0.0-r345380-win32.exe -FileName llvm-installer.exe
+  appveyor DownloadFile https://prereleases.llvm.org/win-snapshots/LLVM-9.0.0-r357435-win32.exe -FileName llvm-installer.exe
 )
 if "%CLANG_VERSION%"=="ToT" (
     START /WAIT llvm-installer.exe /S /D=C:\"Program Files\LLVM"