[Type checker] Minor crash avoidance tweaks.

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -2478,11 +2478,11 @@ namespace {
     }
 
     Type visitDefaultArgumentExpr(DefaultArgumentExpr *expr) {
-      llvm_unreachable("Already type checked");
+      return expr->getType();
     }
 
     Type visitCallerDefaultArgumentExpr(CallerDefaultArgumentExpr *expr) {
-      llvm_unreachable("Already type checked");
+      return expr->getType();
     }
 
     Type visitApplyExpr(ApplyExpr *expr) {

lib/Sema/ConstraintSystem.cpp

@@ -2880,6 +2880,9 @@ ConstraintSystem::getArgumentInfoLocator(ConstraintLocator *locator) {
 
 Optional<ConstraintSystem::ArgumentInfo>
 ConstraintSystem::getArgumentInfo(ConstraintLocator *locator) {
+  if (!locator)
+    return None;
+
   if (auto *infoLocator = getArgumentInfoLocator(locator)) {
     auto known = ArgumentInfos.find(infoLocator);
     if (known != ArgumentInfos.end())