Fixes a race condition in function instantiation.

Previously, if the same function was being concurrently instantiated
and released:

1. Thread one could begin to instantiate the function, determine
   that it already existed in the runtime, then be preempted.
2. Thread two could release the handle on the function, causing it to
   be freed and removed from the `FunctionLibraryRuntime::items_` map.
3. Thread one could then incorrectly assume that the function still
   existed, and fail to find it in the `FunctionLibraryRuntime::items_`
   map, causing a segfault when it attempted to increment the refcount
   on an uninitialized object.

PiperOrigin-RevId: 188661500