[llvm-extract] Support extracting basic blocks

Summary:
Currently, there is no way to extract a basic block from a function easily. This patch
extends llvm-extract to extract the specified basic block(s).

Reviewers: loladiro, rafael, bogner

Reviewed By: bogner

Subscribers: hintonda, mgorny, qcolombet, llvm-commits

Differential Revision: https://reviews.llvm.org/D41638

llvm-svn: 323266