[GSB] Separate out the kind of DelayedRequirement.

DelayedRequirements were using the general RequirementKind, which was
at best an approximation (we smoothed over the difference between
Superclass and Conformance requirements). Split out the Kind into its
own type, so we can extend it with GSB-specific kinds.

Author: DougGregor

include/swift/AST/GenericSignatureBuilder.h

@@ -1678,7 +1678,19 @@ class GenericSignatureBuilder::PotentialArchetype {
 /// Describes a requirement whose processing has been delayed for some reason.
 class GenericSignatureBuilder::DelayedRequirement {
 public:
-  RequirementKind kind;
+  enum Kind {
+    /// A type requirement, which may be a conformance or a superclass
+    /// requirement.
+    Type,
+
+    /// A layout requirement.
+    Layout,
+
+    /// A same-type requirement.
+    SameType,
+  };
+
+  Kind kind;
   UnresolvedType lhs;
   RequirementRHS rhs;
   FloatingRequirementSource source;

lib/AST/GenericSignatureBuilder.cpp

@@ -1209,9 +1209,25 @@ ConstraintResult GenericSignatureBuilder::handleUnresolvedRequirement(
                                    FloatingRequirementSource source,
                                    UnresolvedHandlingKind unresolvedHandling) {
   switch (unresolvedHandling) {
-  case UnresolvedHandlingKind::GenerateConstraints:
-    Impl->DelayedRequirements.push_back({kind, lhs, rhs, source});
+  case UnresolvedHandlingKind::GenerateConstraints: {
+    DelayedRequirement::Kind delayedKind;
+    switch (kind) {
+    case RequirementKind::Conformance:
+    case RequirementKind::Superclass:
+      delayedKind = DelayedRequirement::Type;
+      break;
+
+    case RequirementKind::Layout:
+      delayedKind = DelayedRequirement::Layout;
+      break;
+
+    case RequirementKind::SameType:
+      delayedKind = DelayedRequirement::SameType;
+      break;
+    }
+    Impl->DelayedRequirements.push_back({delayedKind, lhs, rhs, source});
     return ConstraintResult::Resolved;
+  }
 
   case UnresolvedHandlingKind::ReturnUnresolved:
     return ConstraintResult::Unresolved;
@@ -4113,20 +4129,19 @@ void GenericSignatureBuilder::processDelayedRequirements() {
       // Reprocess the delayed requirement.
       ConstraintResult reqResult;
       switch (req.kind) {
-      case RequirementKind::Conformance:
-      case RequirementKind::Superclass:
+      case DelayedRequirement::Type:
         reqResult = addTypeRequirement(
                        req.lhs, asUnresolvedType(req.rhs), req.source,
                        UnresolvedHandlingKind::ReturnUnresolved);
         break;
 
-      case RequirementKind::Layout:
+      case DelayedRequirement::Layout:
         reqResult = addLayoutRequirement(
                            req.lhs, req.rhs.get<LayoutConstraint>(), req.source,
                            UnresolvedHandlingKind::ReturnUnresolved);
         break;
 
-      case RequirementKind::SameType:
+      case DelayedRequirement::SameType:
         reqResult = addSameTypeRequirement(
                                req.lhs, asUnresolvedType(req.rhs), req.source,
                                UnresolvedHandlingKind::ReturnUnresolved);