azure: Remove MSVC tools from PATH for clang-cl job

Add Windows SDK tools to PATH (for rc).  This requires INCLUDE to be set.

lld-link doesn't seem to default to searching the correct directories, so
LIB needs to be set.

The easiest way to get the correct setting of INLCUDE and LIB env vars seems
to be to import the VS env vars, then revert the PATH.

ci/azure-steps.yml

@@ -48,6 +48,7 @@ steps:
      # add downloads to PATH
      $env:Path = "$env:SYSTEM_WORKFOLDER;$env:Path"
 
+     $origPath = $env:Path
      # import visual studio variables
      if ($env:compiler -eq 'msvc2015') {
        $vsver = $env:compiler.Replace('msvc', '')
@@ -59,14 +60,20 @@ steps:
      Install-Module VSSetup -Scope CurrentUser
      Import-VisualStudioVars -VisualStudioVersion $vsver -Architecture $(arch)
 
-     # install llvm for clang-cl builds
-     # (note that we use LIB, WINDRES from VS, so leave those in PATH)
      if ($env:compiler -eq 'clang-cl') {
+       # drop visual studio from PATH
+       # (but leave INCLUDE, LIB and WindowsSdkDir environment variables set)
+       $env:Path = $origPath
+
+       # install llvm for clang-cl builds
        DownloadFile -Source 'http://releases.llvm.org/7.0.0/LLVM-7.0.0-win64.exe' -Destination LLVM-7.0.0-win64.exe
        Start-Process .\LLVM-7.0.0-win64.exe -ArgumentList '/S' -Wait
        $env:Path = "C:\Program Files\LLVM\bin;$env:Path"
        $env:CC = "clang-cl"
        $env:CXX = "clang-cl"
+
+       # and use Windows SDK tools
+       $env:Path = "$env:WindowsSdkDir\bin\$env:Arch;$env:Path"
      }
 
      # add .NET framework tools to path for resgen for C# tests