Create the dir build props / targets structure

This sets up the repository with all of the possible hooks for
Directory.Build.props / targets files. Now that this structure is in
place we can begin migrating our build over to it.

Directory.Build.props

+<!-- Copyright (c)  Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information. -->
+<Project>
+
+</Project>
+

Directory.Build.targets

+<!-- Copyright (c)  Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information. -->
+<Project>
+
+</Project>

build/Targets/AfterCommonTargets.targets

+<!-- Copyright (c)  Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information. -->
+<Project>
+
+</Project>

build/Targets/BeforeCommonTargets.targets

+<!-- Copyright (c)  Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information. -->
+<Project>
+
+</Project>
+

build/Targets/Settings.props

@@ -150,6 +150,9 @@
          This avoids a couple of issues in al.exe (see https://github.com/dotnet/roslyn/issues/23190 and https://github.com/dotnet/roslyn/issues/23191
          for details on those issues). -->
     <GenerateSatelliteAssembliesForCore>true</GenerateSatelliteAssembliesForCore>
+
+    <CustomBeforeMicrosoftCommonTargets>$(MSBuildThisFileDirectory)\BeforeCommonTargets.targets</CustomBeforeMicrosoftCommonTargets>
+    <CustomAfterMicrosoftCommonTargets>$(MSBuildThisFileDirectory)\AfterCommonTargets.targets</CustomAfterMicrosoftCommonTargets>
   </PropertyGroup>
 
   <!-- Windows specific settings -->