[AST] TypeSubstitution: Add a nullptr check before attempting to use inContext

This fixes a crash in `ReplaceOpaqueTypesWithUnderlyingTypes::operator()`
that I couldn't properly reduce because it has to do with `SwiftDeclCollector`.

`ReplaceOpaqueTypesWithUnderlyingTypes::operator()` has the check
already and `canSubstituteTypeInto` expects `inContext` to be
non-null.

Resolves: rdar://156896331

Author: xedin

lib/AST/TypeSubstitution.cpp

@@ -1157,7 +1157,8 @@ operator()(InFlightSubstitution &IFS, Type maybeOpaqueType,
   auto inContext = this->getContext();
   auto isContextWholeModule = this->isWholeModule();
   auto contextExpansion = this->contextExpansion;
-  if (partialSubstTy.findIf(
+  if (inContext &&
+      partialSubstTy.findIf(
           [inContext, substitutionKind, isContextWholeModule,
           contextExpansion](Type t) -> bool {
             if (!canSubstituteTypeInto(t, inContext, substitutionKind,