[llvm-extract] Add option for recursive extraction

Summary:
Particularly, with --delete, this can be very useful for testing
new optimizations on some hotspots, without having to run it on the whole
application. E.g. as such:
```
llvm-extract app.bc --recursive --rfunc .*hotspot.* > hotspot.bc
llvm-extract app.bc --recursive --delete --rfunc .*hotspot.* > residual.bc
llc -filetype=obj residual.bc > residual.o
llc -filetype=obj hotspot.bc > hotspot.o
cc -o app residual.o hotspot.o
```

Reviewed By: davide
Differential Revision: https://reviews.llvm.org/D31722

llvm-svn: 299706