Improve diagnoses of generic specializations

Always add constraints, find fixes during simplify.
New separate fix for allow generic function specialization.
Improve parse heuristic for isGenericTypeDisambiguatingToken.
Degrade concrete type specialization fix to warning for macros.

lib/Sema/CSSimplify.cpp

@@ -13950,6 +13950,8 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
 
   // Bail out if we haven't selected an overload yet.
   auto simplifiedBoundType = simplifyType(type1, flags);
+  if (simplifiedBoundType->isPlaceholder())
+    return SolutionKind::Solved;
   if (simplifiedBoundType->isTypeVariableOrMember())
     return formUnsolved();
 
@@ -14042,13 +14044,15 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
     }
   }
 
-  if (!decl->getAsGenericContext())
-    return SolutionKind::Error;
-
   auto genericParams = getGenericParams(decl);
-  if (!genericParams) {
-    // FIXME: Record an error here that we're ignoring the parameters.
-    return SolutionKind::Solved;
+  if (!decl->getAsGenericContext() || !genericParams) {
+    // Allow concrete macros to have specializations with just a warning.
+    return recordFix(AllowConcreteTypeSpecialization::create(
+               *this, type1, decl, getConstraintLocator(locator),
+               isa<MacroDecl>(decl) ? FixBehavior::DowngradeToWarning
+                                    : FixBehavior::Error))
+               ? SolutionKind::Error
+               : SolutionKind::Solved;
   }
 
   // Map the generic parameters we have over to their opened types.
@@ -14081,12 +14085,14 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
     }
   }
 
-  if (openedGenericParams.empty()) {
+  // FIXME: We could support explicit function specialization.
+  if (openedGenericParams.empty() ||
+      (isa<AbstractFunctionDecl>(decl) && !hasParameterPack)) {
     if (!shouldAttemptFixes())
       return SolutionKind::Error;
 
-    return recordFix(AllowConcreteTypeSpecialization::create(
-               *this, type1, getConstraintLocator(locator)))
+    return recordFix(AllowGenericFunctionSpecialization::create(
+               *this, decl, getConstraintLocator(locator)))
                ? SolutionKind::Error
                : SolutionKind::Solved;
   }
@@ -15217,6 +15223,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowAssociatedValueMismatch:
   case FixKind::GenericArgumentsMismatch:
   case FixKind::AllowConcreteTypeSpecialization:
+  case FixKind::AllowGenericFunctionSpecialization:
   case FixKind::IgnoreGenericSpecializationArityMismatch:
   case FixKind::IgnoreKeyPathSubscriptIndexMismatch:
   case FixKind::AllowMemberRefOnExistential: {