[code-completion] Fix crash in typeCheckConstructorBodyUntil with null nominal context

With erroneous code, such as in code-completion, it's not safe to assume
that the constructor is inside a nominal decl context.

rdar://problem/28867794

Author: benlangmuir

lib/Sema/TypeCheckStmt.cpp

@@ -1403,7 +1403,7 @@ bool TypeChecker::typeCheckConstructorBodyUntil(ConstructorDecl *ctor,
   // Determine whether we need to introduce a super.init call.
   auto nominalDecl = ctor->getDeclContext()
     ->getAsNominalTypeOrNominalTypeExtensionContext();
-  ClassDecl *ClassD = dyn_cast<ClassDecl>(nominalDecl);
+  ClassDecl *ClassD = dyn_cast_or_null<ClassDecl>(nominalDecl);
   bool wantSuperInitCall = false;
   if (ClassD) {
     bool isDelegating = false;

validation-test/IDE/crashers/040-swift-typechecker-typecheckconstructorbodyuntil.swift

@@ -0,0 +1,2 @@
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=A -source-filename=%s
+t{extension{init(){#^A^#