Revert r277556. Add -lowertypetests-bitsets-level to control bitsets generation

Summary:
We don't currently need this facility for CFI. Disabling individual hot methods proved
to be a better strategy in Chrome.

Also, the design of the feature is suboptimal, as pointed out by Peter Collingbourne.

Reviewers: pcc

Subscribers: kcc

Differential Revision: https://reviews.llvm.org/D24948

llvm-svn: 282461