We added support for the GNUstep ObjC runtime in 0b6264738f. In order to check if the target process uses
GNUstep we run an expensive symbol lookup in `CreateInstance()`. This turned out to cause a heavy performance
regression for non-GNUstep inferiors.
This patch puts a cheaper check in front, so that the vast majority of requests should return early. This
should fix the symptom for the moment. The conceptual question remains: Why does `LanguageRuntime::FindPlugin`
invoke `create_callback` for each available runtime unconditionally in every `Process::ModulesDidLoad`?
Reviewed By: jasonmolenda, jingham, bulbazord
Differential Revision: https://reviews.llvm.org/D158205
af2eb83830