[llvm-extract] Expose the group extraction feature of the BlockExtractor

This patch extends the `-bb` option to be able to use the group
extraction feature from the BlockExtractor.
In particular, `-bb=func:bb` is modified to support a list of basic
blocks per function: `-bb=func:bb1[;bb2...]` that will be extracted
together if at all possible (region must be single entry.)

Differential Revision: https://reviews.llvm.org/D60973

llvm-svn: 359464