[Sanitizer] Refactor sanitizer options in LangOptions.

Get rid of ugly SanitizerOptions class thrust into LangOptions:
* Make SanitizeAddressFieldPadding a regular language option,
  and rely on default behavior to initialize/reset it.
* Make SanitizerBlacklistFile a regular member LangOptions.
* Introduce the helper class "SanitizerSet" to represent the
  set of enabled sanitizers and make it a member of LangOptions.
  It is exactly the entity we want to cache and modify in CodeGenFunction,
  for instance. We'd also be able to reuse SanitizerSet in
  CodeGenOptions for storing the set of recoverable sanitizers,
  and in the Driver to represent the set of sanitizers
  turned on/off by the commandline flags.

No functionality change.