[ClangImporter] Add a bunch of non-null assertions.

rdar://problem/23985987 has a crash report that implies that a null
snuck in here at some point in the past. All the refactoring that's
happened since then probably means it can't happen any more, but
assertions are always good, right?

Author: jrose-apple

lib/ClangImporter/ImportDecl.cpp

@@ -4254,6 +4254,7 @@ namespace {
                                        ArrayRef<ProtocolDecl *> protocols,
                                        SmallVectorImpl<Decl *> &members,
                                        ASTContext &Ctx) {
+      assert(dc);
       const clang::ObjCInterfaceDecl *interfaceDecl = nullptr;
       const ClangModuleUnit *declModule;
       const ClangModuleUnit *interfaceModule;
@@ -4907,6 +4908,8 @@ namespace {
 
     Decl *VisitObjCPropertyDecl(const clang::ObjCPropertyDecl *decl,
                                 DeclContext *dc) {
+      assert(dc);
+
       auto name = Impl.importFullName(decl).Imported.getBaseName();
       if (name.empty())
         return nullptr;
@@ -5626,6 +5629,7 @@ Decl *
 ClangImporter::Implementation::importMirroredDecl(const clang::NamedDecl *decl,
                                                   DeclContext *dc,
                                                   ProtocolDecl *proto) {
+  assert(dc);
   if (!decl)
     return nullptr;
 
@@ -5930,6 +5934,7 @@ createUnavailableDecl(Identifier name, DeclContext *dc, Type type,
 
 void
 ClangImporter::Implementation::loadAllMembers(Decl *D, uint64_t unused) {
+  assert(D);
   assert(D->hasClangNode());
   auto clangDecl = cast<clang::ObjCContainerDecl>(D->getClangDecl());