[SubstitutionMap] Handle substitution of non-canonical generic parameters.

SubstitutionMap::lookupSubstitution() assumed that any type parameter it was
given would already be canonical, and fail (return a null Type) when given
a type parameter that was equivalent to another type parameter but is
not itself canonical. Canonicalize, recursive, and record the replacement
type appropriately.

lib/AST/SubstitutionMap.cpp

@@ -283,8 +283,19 @@ Type SubstitutionMap::lookupSubstitution(CanSubstitutableType type) const {
     return replacementType;
   }
 
-  // Not known.
-  return Type();
+  // The generic parameter may not be canonical. Retrieve the canonical
+  // type, which will be dependent.
+  CanType canonicalType = genericSig->getCanonicalTypeInContext(genericParam);
+
+  // If nothing changed, we don't have a replacement.
+  if (canonicalType == type) return Type();
+
+  // If we're left with a substitutable type, substitute into that.
+  // First, set the replacement type to an error, to block infinite recursion.
+  replacementType = ErrorType::get(type);
+
+  replacementType = lookupSubstitution(cast<SubstitutableType>(canonicalType));
+  return replacementType;
 }
 
 Optional<ProtocolConformanceRef>