AST: Handle request cycle in TypeBase::getMemberSubstitutions()

I don't have a test case, but it's possible that getGenericSignature()
returns nullptr, even if the declaration has a generic parameter list,
because of a request cycle.

Fixes <rdar://problem/60045501>.

Author: slavapestov

lib/AST/Type.cpp

@@ -4017,8 +4017,7 @@ TypeSubstitutionMap TypeBase::getMemberSubstitutions(
       isa<GenericTypeDecl>(member) ||
       isa<SubscriptDecl>(member)) {
     auto *innerDC = member->getInnermostDeclContext();
-    if (innerDC->isInnermostContextGeneric()) {
-      auto sig = innerDC->getGenericSignatureOfContext();
+    if (auto sig = innerDC->getGenericSignatureOfContext()) {
       for (auto param : sig->getInnermostGenericParams()) {
         auto *genericParam = param->getCanonicalType()
             ->castTo<GenericTypeParamType>();