Add -lowertypetests-bitsets-level to control bitsets generation.

Summary:
Sometimes, bitsets could get really large (>300k entries) and
we might want to drop a check, as it would have a too much cost.

Adding a flag to control how much penalty are we willing to pay
for bitsets.

Reviewers: kcc

Differential Revision: https://reviews.llvm.org/D23088

llvm-svn: 277556