Permit type overlap between System.Memory and S.P.Corelib on netcoreapp2.0

In netcoreapp2.0 the types in CoreLib were experimental and should not be exposed by System.Memory.

Author: ericstj

pkg/test/frameworkSettings/netcoreapp2.0/settings.targets

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup Condition="'$(RuntimeIdentifier)' != ''">
+    <!-- NETCoreApp2.0 had experimental versions of Span and ReadOnlySpan 
+         which we don't match the final API nor were they ship-quality -->
+    <IgnoredTypes Include="System.ReadOnlySpan`1" />
+    <IgnoredTypes Include="System.Span`1" />
+  </ItemGroup>
+</Project>