Use PolySharp to generate polyfills

Author: Sergio0694

src/CommunityToolkit.Common/Attributes/NotNullIfNotNullAttribute.cs

@@ -19,4 +19,13 @@
     <DefineConstants>NETSTANDARD2_1_OR_GREATER</DefineConstants>
   </PropertyGroup>
 
+  <!-- Necessary polyfills -->
+  <PropertyGroup>
+    <PolySharpIncludeGeneratedTypes>
+      System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute;
+      System.Diagnostics.CodeAnalysis.NotNullWhenAttribute;
+      System.Runtime.CompilerServices.SkipLocalsInitAttribute;
+    </PolySharpIncludeGeneratedTypes>
+  </PropertyGroup>
+
 </Project>

src/CommunityToolkit.Common/Attributes/NotNullWhenAttribute.cs

@@ -34,14 +34,26 @@
       </ItemGroup>
     </When>
 
-    <!-- Enable trimming support on .NET 6 -->
+    <!-- For the .NET 6 target, simply define NETSTANDARD2_1_OR_GREATER too -->
     <When Condition="'$(TargetFramework)' == 'net6.0'">
       <PropertyGroup>
         <DefineConstants>NETSTANDARD2_1_OR_GREATER</DefineConstants>
       </PropertyGroup>
     </When>
   </Choose>
 
+  <!-- Necessary polyfills -->
+  <PropertyGroup>
+    <PolySharpIncludeGeneratedTypes>
+      System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute;
+      System.Diagnostics.CodeAnalysis.DoesNotReturnIfAttribute;
+      System.Diagnostics.CodeAnalysis.NotNullAttribute;
+      System.Diagnostics.StackTraceHiddenAttribute;
+      System.Runtime.CompilerServices.CallerArgumentExpressionAttribute;
+      System.Runtime.CompilerServices.SkipLocalsInitAttribute;
+    </PolySharpIncludeGeneratedTypes>
+  </PropertyGroup>
+
   <ItemGroup>
     <None Update="Generated\Guard.Comparable.Numeric.tt">
       <Generator>TextTemplatingFileGenerator</Generator>

src/CommunityToolkit.Common/Attributes/SkipLocalsInitAttribute.cs

@@ -54,4 +54,13 @@
     </When>
   </Choose>
 
+  <!-- Necessary polyfills -->
+  <PropertyGroup>
+    <PolySharpIncludeGeneratedTypes>
+      System.Diagnostics.CodeAnalysis.NotNullAttribute;
+      System.Diagnostics.CodeAnalysis.NotNullWhenAttribute;
+      System.Runtime.CompilerServices.SkipLocalsInitAttribute;
+    </PolySharpIncludeGeneratedTypes>
+  </PropertyGroup>
+
 </Project>