[CSGen] Mark visitDynamicMemberRefExpr as unreachable to match ExprRewriter and update SanitizeExpr to handle them

We were hitting an unreachable in visitDynamicMemberExpr in ExprRewriter when
re-typechecking during a salvage. Check for these earlier (in CSGen) and update
SanitizeExpr to handle them.

Resolves rdar://problem/39055736

Author: Nathan Hawes

lib/Sema/CSGen.cpp

@@ -1406,9 +1406,7 @@ namespace {
     }
 
     Type visitDynamicMemberRefExpr(DynamicMemberRefExpr *expr) {
-      return addMemberRefConstraints(expr, expr->getBase(),
-                                     expr->getMember().getDecl(),
-                                     /*FIXME:*/FunctionRefKind::DoubleApply);
+      llvm_unreachable("Already typechecked");
     }
 
     virtual Type visitUnresolvedMemberExpr(UnresolvedMemberExpr *expr) {
@@ -3113,6 +3111,17 @@ namespace {
         }
       }
 
+      if (auto *dynamicMember = dyn_cast<DynamicMemberRefExpr>(expr)) {
+        if (auto memberRef = dynamicMember->getMember()) {
+          auto base = skipImplicitConversions(dynamicMember->getBase());
+          return new (TC.Context) MemberRefExpr(base,
+                                                dynamicMember->getDotLoc(),
+                                                memberRef,
+                                                dynamicMember->getNameLoc(),
+                                                expr->isImplicit());
+        }
+      }
+
       if (auto dotIgnored = dyn_cast<DotSyntaxBaseIgnoredExpr>(expr)) {
         // A DotSyntaxBaseIgnoredExpr is a static member reference that has
         // already been type-checked down to a call where the argument doesn't

validation-test/compiler_crashers_2_fixed/0150-rdar39055736.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck %s -verify
+// REQUIRES: objc_interop
+
+import Foundation
+
+@objc class Foo: NSObject {
+  let x: Int = 0
+}
+@objc protocol Bar {
+  @objc optional var foo: Foo {get}
+}
+
+func baz(bar: Bar) {
+  max(bar, bar.foo?.x ?? 0)
+  // expected-error@-1 {{cannot invoke 'max' with an argument list of type '(Bar, Int)'}}
+  // expected-note@-2 {{expected an argument list of type '(T, T)'}}
+}