[embedded] Add explaining comment about location-less SIL functions in PerformanceDiagnostics

Author: kubamracek

lib/SILOptimizer/Mandatory/PerformanceDiagnostics.cpp

@@ -810,6 +810,10 @@ class PerformanceDiagnosticsPass : public SILModuleTransform {
       SmallVector<SILFunction *, 8> constructorsAndDestructors;
 
       for (SILFunction &function : *module) {
+        // There might be SILFunctions without a location, e.g.
+        // swift_readAtKeyPath generated by SILGen for keypaths. It's okay to
+        // skip the ctor/dtor/method detection logic for those, such functions
+        // still end up in the "others" list and are still visited.
         if (function.hasLocation()) {
           auto func =
               function.getLocation().getAsASTNode<AbstractFunctionDecl>();