CompileFeatures: Don't try_compile for language levels with no features

Previously Compilers always had to run a try_compile to determine what
language level each feature mapped to. Now we can skip the try_compile
when a language level has no features.

Modules/Internal/FeatureTesting.cmake

@@ -71,10 +71,20 @@ endmacro()
 
 macro(_record_compiler_features_c std)
   list(APPEND CMAKE_C${std}_COMPILE_FEATURES c_std_${std})
-  _record_compiler_features(C "${CMAKE_C${std}_STANDARD_COMPILE_OPTION}" CMAKE_C${std}_COMPILE_FEATURES)
+
+  get_property(lang_level_has_features GLOBAL PROPERTY CMAKE_C${std}_KNOWN_FEATURES)
+  if(lang_level_has_features)
+    _record_compiler_features(C "${CMAKE_C${std}_STANDARD_COMPILE_OPTION}" CMAKE_C${std}_COMPILE_FEATURES)
+  endif()
+  unset(lang_level_has_features)
 endmacro()
 
 macro(_record_compiler_features_cxx std)
   list(APPEND CMAKE_CXX${std}_COMPILE_FEATURES cxx_std_${std})
-  _record_compiler_features(CXX "${CMAKE_CXX${std}_STANDARD_COMPILE_OPTION}" CMAKE_CXX${std}_COMPILE_FEATURES)
+
+  get_property(lang_level_has_features GLOBAL PROPERTY CMAKE_CXX${std}_KNOWN_FEATURES)
+  if(lang_level_has_features)
+    _record_compiler_features(CXX "${CMAKE_CXX${std}_STANDARD_COMPILE_OPTION}" CMAKE_CXX${std}_COMPILE_FEATURES)
+  endif()
+  unset(lang_level_has_features)
 endmacro()