VERIFY_INTERFACE_HEADER_SETS: Add IWYU pragma: associated to verification file

Source/cmGeneratorTarget.cxx

@@ -8760,7 +8760,11 @@ std::string cmGeneratorTarget::GenerateHeaderSetVerificationFile(
 
   cmGeneratedFileStream fout(filename);
   fout.SetCopyIfDifferent(true);
-  fout << "#include <" << headerFilename << ">\n";
+  // IWYU pragma: associated allows include what you use to
+  // consider the headerFile as part of the entire language
+  // unit within include-what-you-use and as a result allows
+  // one to get IWYU advice for headers :)
+  fout << "#include <" << headerFilename << "> // IWYU pragma: associated\n";
   fout.close();
 
   return filename;

Tests/RunCMake/VerifyHeaderSets/VerifyHeaderSets-check.cmake

@@ -9,7 +9,7 @@ function(check_file target filename)
   if(filename MATCHES "^(.*)(\\.[a-z]+)$")
     set(header_filename "${CMAKE_MATCH_1}")
   endif()
-  set(expected_contents "#include <${header_filename}>\n")
+  set(expected_contents "#include <${header_filename}> // IWYU pragma: associated\n")
   file(READ "${full_filename}" actual_contents)
 
   if(NOT actual_contents STREQUAL expected_contents)