[Sanitizer] Refactor SanitizerArgs parsing in Driver.

Remove flag parsing details from the public header.
Use SanitizerSet to represent the set of enabled sanitizers.
Cleanup the implementation: update the comments to
reflect reality, remove dead code.

No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221968 91177308-0d34-0410-b5e6-96231b3b80d8