[ConstraintSystem] Add a KeyPathComponentResult locator path element

We’ll use this instead of the function result, except for subscript components, which are already structured similarly to subscript expressions.

Author: beccadax

lib/Sema/CSGen.cpp

@@ -2977,7 +2977,7 @@ namespace {
         auto memberLocator = CS.getConstraintLocator(
             locator, ConstraintLocator::PathElement::getKeyPathComponent(i));
         auto resultLocator = CS.getConstraintLocator(
-            memberLocator, ConstraintLocator::FunctionResult);
+            memberLocator, ConstraintLocator::KeyPathComponentResult);
 
         switch (auto kind = component.getKind()) {
         case KeyPathExpr::Component::Kind::Invalid:

lib/Sema/ConstraintLocator.cpp

@@ -81,6 +81,7 @@ void ConstraintLocator::Profile(llvm::FoldingSetNodeID &id, Expr *anchor,
     case SynthesizedArgument:
     case KeyPathRoot:
     case KeyPathValue:
+    case KeyPathComponentResult:
       if (unsigned numValues = numNumericValuesInPathElement(elt.getKind())) {
         id.AddInteger(elt.getValue());
         if (numValues > 1)
@@ -332,19 +333,23 @@ void ConstraintLocator::dump(SourceManager *sm, raw_ostream &out) {
       break;
 
     case SynthesizedArgument:
-      out << " synthesized argument #" << llvm::utostr(elt.getValue());
+      out << "synthesized argument #" << llvm::utostr(elt.getValue());
       break;
 
     case KeyPathDynamicMember:
-      out << " keypath dynamic member lookup";
+      out << "key path dynamic member lookup";
       break;
 
     case KeyPathRoot:
-      out << " keypath root";
+      out << "key path root";
       break;
 
     case KeyPathValue:
-      out << " keypath value";
+      out << "key path value";
+      break;
+
+    case KeyPathComponentResult:
+      out << "key path component result";
       break;
     }
   }

lib/Sema/ConstraintLocator.h

@@ -133,6 +133,8 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     KeyPathRoot,
     /// The value of a keypath
     KeyPathValue,
+    /// The result type of a key path component. Not used for subscripts.
+    KeyPathComponentResult,
   };
 
   /// Determine the number of numeric values used for the given path
@@ -165,6 +167,7 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     case ContextualType:
     case KeyPathRoot:
     case KeyPathValue:
+    case KeyPathComponentResult:
       return 0;
 
     case OpenedGeneric:
@@ -233,6 +236,7 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     case KeyPathDynamicMember:
     case KeyPathRoot:
     case KeyPathValue:
+    case KeyPathComponentResult:
       return 0;
 
     case FunctionArgument: