Avoid x-targeting perf impact on vcxproj

Targeted mitigation for #1276.

VC++ noticed a dramatic regression in solution-load performance related to
the time cost of ResolveProjectReferences. That affects all projects, but
we can safely assume that a .vcxproj is NOT using cross-targeting, while
it's impossible to detect from the outside whether a .csproj is.

This commit avoids the regression for C++ projects by just not calling the
MSBuild task again for .vcxproj references.

src/XMakeTasks/Microsoft.Common.CurrentVersion.targets

@@ -1504,7 +1504,12 @@ Copyright (C) Microsoft Corporation. All rights reserved.
 
     ======================================================================================
   -->
-  <Target Name="_GetProjectReferenceTargetFrameworkProperties" Outputs="%(_MSBuildProjectReferenceExistent.Identity)">
+  <!-- As a mitigation for https://github.com/Microsoft/msbuild/issues/1276
+       .vcxproj is treated specially to avoid the cost of double-evaluating
+       for the easily-identifiable-as-not-cross-targeting C++ projects. -->
+  <Target Name="_GetProjectReferenceTargetFrameworkProperties"
+          Outputs="%(_MSBuildProjectReferenceExistent.Identity)"
+          Condition="'%(_MSBuildProjectReferenceExistent.Extension)' != '.vcxproj'">
     <MSBuild
         Projects="%(_MSBuildProjectReferenceExistent.Identity)"
         Targets="GetTargetFrameworkProperties"