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

@@ -4018,14 +4018,15 @@ TypeSubstitutionMap TypeBase::getMemberSubstitutions(
       isa<SubscriptDecl>(member)) {
     auto *innerDC = member->getInnermostDeclContext();
     if (innerDC->isInnermostContextGeneric()) {
-      auto sig = innerDC->getGenericSignatureOfContext();
-      for (auto param : sig->getInnermostGenericParams()) {
-        auto *genericParam = param->getCanonicalType()
-            ->castTo<GenericTypeParamType>();
-        substitutions[genericParam] =
-          (genericEnv
-           ? genericEnv->mapTypeIntoContext(param)
-           : param);
+      if (auto sig = innerDC->getGenericSignatureOfContext()) {
+        for (auto param : sig->getInnermostGenericParams()) {
+          auto *genericParam = param->getCanonicalType()
+              ->castTo<GenericTypeParamType>();
+          substitutions[genericParam] =
+            (genericEnv
+             ? genericEnv->mapTypeIntoContext(param)
+             : param);
+        }
       }
     }
   }