4119: Cache proc-macro dlls r=matklad a=edwin0cheng
This PR try to fix a deadlock in proc-macro srv by not unloading dlls.
Currently we load and unload dlls for each request, however rustc TLS is leaky , such that if we do it a lot of times, all TLS index will be consumed and it will be deadlocked inside panic (it is because panic itself is using TLS too).
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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