Depwarn caller fix

[c42f]

This fixes the core problem of #25130, namely that on occasion - and especially at top level scope - the internal dispatch or interpreter machinery was being blamed as the caller of a deprecated function.

Along the way I also noticed what was arguably a bug in global_logger(logger), so that's also fixed to more usefully return the old logger rather than the replacement.

[c42f]

CI failure seems unrelated... if potentially serious

      From worker 8:	julia: /home/circleci/project/deps/srccache/llvm-3.9.1/lib/MC/MCContext.cpp:118: llvm::MCSymbol* llvm::MCContext::getOrCreateSymbol(const llvm::Twine&): Assertion `!NameRef.empty() && "Normal symbols cannot be unnamed!"' failed.
      From worker 8:	
      From worker 8:	signal (6): Aborted

[StefanKarpinski]

cc @vtjnash ^

[c42f]

Can this be merged? It's a small bugfix, CI has passed, and I want to build some other things on top of it.

[StefanKarpinski]

Should I squash or not?

[c42f]

I think not, the two commits here represent distinct changes.