Add testcase for @_implements parser diagnostics.

Author: graydon

test/attr/attr_implements_bad_parse.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -parse -verify %s
+
+struct S0 {
+  @_implements(1, Foo) // expected-error {{expected type}}
+  func f() { }
+}
+
+struct S1 {
+  @_implements(NeedsF0, 1) // expected-error {{expected a member name as second parameter in '_implements' attribute}}
+  func f() { }
+}
+
+struct S2 {
+  @_implements(NeedsF0) // expected-error {{expected ',' in '_implements' attribute}}
+  func f() { }
+}