[Sema] fix typevar attr for subscript

Author: omochi

lib/Sema/CSGen.cpp

@@ -1060,7 +1060,8 @@ namespace {
 
       // Add the member constraint for a subscript declaration.
       // FIXME: weak name!
-      auto memberTy = CS.createTypeVariable(memberLocator, TVO_CanBindToNoEscape);
+      auto memberTy = CS.createTypeVariable(
+          memberLocator, TVO_CanBindToLValue | TVO_CanBindToNoEscape);
 
       // FIXME: synthesizeMaterializeForSet() wants to statically dispatch to
       // a known subscript here. This might be cleaner if we split off a new

test/attr/attr_dynamic_member_lookup.swift

@@ -619,7 +619,7 @@ struct WithTrailingClosure {
 
 func keypath_with_trailing_closure_subscript(_ ty: inout SubscriptLens<WithTrailingClosure>) {
   _ = ty[0] { 42 } // expected-error {{subscript index of type '() -> Int' in a key path must be Hashable}}
-  _ = ty[0] { 42 } = 0
+  _ = ty[0] { 42 } = 0 // expected-error {{cannot assign through subscript: subscript is get-only}}
   // expected-error@-1 {{subscript index of type '() -> Int' in a key path must be Hashable}}
   _ = ty[] { 42 }  // expected-error {{subscript index of type '() -> Int' in a key path must be Hashable}}
   _ = ty[] { 42 } = 0 // expected-error {{subscript index of type '() -> Int' in a key path must be Hashable}}