[lit] Support sharding testsuites, for parallel execution.

Summary:
This change equips lit.py with two new options, --num-shards=M and
--run-shard=N (set by default from env vars LIT_NUM_SHARDS and LIT_RUN_SHARD).

The options must be used together, and N must be in 1..M.

Together these options effect only test selection: they partition the testsuite
into M equal-sized "shards", then select only the Nth shard. They can be used
in a cluster of test machines to achieve a very crude (static) form of
parallelism, with minimal configuration work.

Reviewers: modocache, ddunbar

Reviewed By: ddunbar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28789

llvm-svn: 292417