[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