IRGen: swift_getFunctionTypeMetadata is ReadOnly

It only accesses the type arguments passed indirectly via an array

include/swift/Runtime/RuntimeFunctions.def

@@ -746,7 +746,7 @@ FUNCTION(GetFunctionMetadata, swift_getFunctionTypeMetadata, C_CC,
               TypeMetadataPtrTy->getPointerTo(0),
               Int32Ty->getPointerTo(0),
               TypeMetadataPtrTy),
-         ATTRS(NoUnwind))
+         ATTRS(NoUnwind, ReadOnly))
 
 // Metadata *swift_getFunctionTypeMetadata0(unsigned long flags,
 //                                          const Metadata *resultMetadata);