SourceKit: Fix indexing crash with protocol typealiases

The root cause is that NormalProtocolConformance::forEachValueWitness()
needs to skip protocol members that are not requirements.

Otherwise we end up passing such a non-requirement member down to
NormalProtocolConformance::getWitness() and hit an assert when we
cannot find it.

It looks like this code path was only ever hit from SourceKit.
The fix moves TypeChecker::isRequirement() to a method on ValueDecl,
and calls it in the right places.

Fixes <https://bugs.swift.org/browse/SR-3815>.

lib/AST/ProtocolConformance.cpp

@@ -321,6 +321,8 @@ void NormalProtocolConformance::setTypeWitness(
 Witness NormalProtocolConformance::getWitness(ValueDecl *requirement,
                                               LazyResolver *resolver) const {
   assert(!isa<AssociatedTypeDecl>(requirement) && "Request type witness");
+  assert(requirement->isProtocolRequirement() && "Not a requirement");
+
   if (Resolver)
     resolveLazyInfo();