[GenericEnvironment] API to iterate pack element

The function `forEachPackElementBinding` found "interesting"
GenericTypeParamTypes, transformed each, and called back.  Here the work
of finding such "interesting" types is pulled out into a separate
function `forEachPackElementGenericTypeParam` through which
`forEachPackElementBinding` now factors.

Author: nate-chandler

include/swift/AST/GenericEnvironment.h

@@ -200,6 +200,9 @@ class alignas(1 << DeclAlignInBits) GenericEnvironment final
   void forEachPackElementArchetype(
           llvm::function_ref<void(ElementArchetypeType*)> function) const;
 
+  void forEachPackElementGenericTypeParam(
+      llvm::function_ref<void(GenericTypeParamType *)> function) const;
+
   using PackElementBindingCallback =
     llvm::function_ref<void(ElementArchetypeType *elementType,
                             PackType *packSubstitution)>;

lib/AST/GenericEnvironment.cpp

@@ -141,13 +141,12 @@ void GenericEnvironment::forEachPackElementArchetype(
   }
 }
 
-void GenericEnvironment::forEachPackElementBinding(
-    PackElementBindingCallback function) const {
+void GenericEnvironment::forEachPackElementGenericTypeParam(
+    llvm::function_ref<void(GenericTypeParamType *)> function) const {
   auto sig = getGenericSignature();
   auto shapeClass = getOpenedElementShapeClass();
   auto packElements = sig.getInnermostGenericParams();
   auto packElementDepth = packElements.front()->getDepth();
-  auto elementIt = packElements.begin();
 
   // Each parameter pack in the outer generic parameters has
   // a corresponding pack element parameter at the innermost
@@ -164,13 +163,23 @@ void GenericEnvironment::forEachPackElementBinding(
     if (!sig->haveSameShape(genericParam, shapeClass->mapTypeOutOfContext()))
       continue;
 
+    function(genericParam);
+  }
+}
+
+void GenericEnvironment::forEachPackElementBinding(
+    PackElementBindingCallback function) const {
+  auto sig = getGenericSignature();
+  auto packElements = sig.getInnermostGenericParams();
+  auto elementIt = packElements.begin();
+  forEachPackElementGenericTypeParam([&](auto *genericParam) {
     assert(elementIt != packElements.end());
     auto *elementArchetype =
         mapTypeIntoContext(*elementIt++)->castTo<ElementArchetypeType>();
-    auto *packSubstitution =
-        maybeApplyOuterContextSubstitutions(genericParam)->castTo<PackType>();
+    auto *packSubstitution = maybeApplyOuterContextSubstitutions(genericParam)
+                                 ->template castTo<PackType>();
     function(elementArchetype, packSubstitution);
-  }
+  });
 
   assert(elementIt == packElements.end());
 }