[CSDiag] Fix crash related to failures in contextual type requirements

If failed constraint mentions member declaration which is not
generic, it means that generic requirements came from context
and should not be diagnosed by `diagnoseUnresolvedDotExprTypeRequirementFailure`.

Resolved: rdar://problem/45511837

Author: xedin

lib/Sema/CSDiag.cpp

@@ -1289,7 +1289,8 @@ diagnoseUnresolvedDotExprTypeRequirementFailure(ConstraintSystem &cs,
   // If we actually resolved the member to use, use it.
   auto loc = cs.getConstraintLocator(UDE, ConstraintLocator::Member);
   auto *member = cs.findResolvedMemberRef(loc);
-  if (!member)
+  // If the problem is contextual it's diagnosed elsewhere.
+  if (!member || !member->getAsGenericContext())
     return false;
 
   auto req = member->getAsGenericContext()

test/Constraints/rdar45511837.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify %s
+// REQUIRES: objc_interop
+
+import Foundation
+
+protocol A: RawRepresentable {}
+
+extension A {
+  static func +(lhs: RawValue, rhs: Self) -> Self {
+    fatalError()
+  }
+}
+
+class Foo<Bar: NSObject> {
+  var foobar: Bar {
+    fatalError()
+  }
+
+  lazy var foo: () -> Void = {
+    // TODO: improve diagnostic message
+    _ = self.foobar + nil // expected-error {{'Foo<Bar>' requires that 'Bar' inherit from 'NSObject'}}
+  }
+}