CMP0171: Improve diagnostic context

When issuing the policy warning that `codegen` is a reserved target
name, use the diagnostic context in which the target was declared,
rather than the root context (which cannot be affected in script).

Source/cmGlobalGenerator.cxx

@@ -3101,7 +3101,7 @@ void cmGlobalGenerator::ReserveGlobalTargetCodegen()
 
   switch (policyStatus) {
     case cmPolicies::WARN:
-      this->GetCMakeInstance()->IssueDiagnostic(
+      tgt->GetMakefile()->IssueDiagnostic(
         cmDiagnostics::CMD_AUTHOR,
         cmStrCat(cmPolicies::GetPolicyWarning(cmPolicies::CMP0171), '\n',
                  "The target name \"codegen\" is reserved."),

Tests/RunCMake/CMP0171/CMP0171-WARN-QUIET.cmake

@@ -0,0 +1 @@
+add_subdirectory("CMP0171-WARN-QUIET")

Tests/RunCMake/CMP0171/CMP0171-WARN-QUIET/CMakeLists.txt

@@ -0,0 +1,8 @@
+# CMake should warn the user if they have a target named codegen... unless the
+# user also suppresses the diagnostic.
+cmake_diagnostic(SET CMD_AUTHOR IGNORE RECURSE)
+add_custom_target(codegen
+  COMMAND ${CMAKE_COMMAND} -E copy
+    ${CMAKE_CURRENT_SOURCE_DIR}/generated.h.in
+    ${CMAKE_CURRENT_BINARY_DIR}/generated.h
+)

Tests/RunCMake/CMP0171/RunCMakeTest.cmake

@@ -1,6 +1,7 @@
 include(RunCMake)
 
 run_cmake("CMP0171-WARN")
+run_cmake("CMP0171-WARN-QUIET")
 
 run_cmake_with_options(CMP0171-OLD "-DCMAKE_POLICY_DEFAULT_CMP0171=OLD")