[src] Add optional StyleCop validation. (#22208)

This makes it easy to reformat code using StyleCop!

I'll start enabling rules in separate PRs.

src/Directory.Build.props

@@ -13,4 +13,10 @@
 			<NoWarn>$(NoWarn.Replace('CA1416','').Replace('CA1422',''))</NoWarn>
 		</PropertyGroup>
 	</Target>
+	<PropertyGroup Condition="'$(EnableStyleCop)' == 'true'">
+		<CodeAnalysisRuleSet>$(MSBuildThisFileDirectory)/stylecop.ruleset</CodeAnalysisRuleSet>
+	</PropertyGroup>
+	<ItemGroup Condition="'$(EnableStyleCop)' == 'true'">
+		<PackageReference Include="StyleCop.Analyzers" Version="1.2.0-beta.556" />
+	</ItemGroup>
 </Project>

src/Makefile

@@ -668,6 +668,14 @@ analyze:
 		echo "Analyzed $$csproj successfully."; \
 	done
 
+# This target builds the generated project files with StyleCop enabled.
+stylecop-csproj: $(CSPROJECTS) $(foreach platform,$(DOTNET_PLATFORMS),$($(shell echo $(platform) | tr '[:lower:]' '[:upper:]')_DOTNET_PLATFORM_ASSEMBLY_DEPENDENCIES))
+	$(Q) set -e; \
+	for csproj in $(CSPROJECTS); do \
+		echo "Building $$csproj with StyleCop enabled..."; \
+		$(DOTNET) build $$csproj $(DOTNET_BUILD_VERBOSITY) /p:EnableStyleCop=true; \
+		echo "Built $$csproj with StyleCop successfully."; \
+	done
 
 csproj:: $(CSPROJECTS)
 ALL_TARGETS += $(CSPROJECTS)