[libclang] Refactored SharedParsedRegionsStorage

Summary:
Removed the `PPRegionSetTy` typedef because it is only used 3 times, and
obscures code more than it helps.

Renamed SharedParsedRegionsStorage to ThreadSafeParsedRegions, because
that better reflects the reason for this type to exist.

Replaced the `copyTo()` method that had an out parameter with a getter.

Renamed the `merge()` method to `addParsedRegions()`.

Renamed `ParsedSrcLocationsTracker::ParsedRegions` to
`ParsedRegionsSnapshot`, which better reflects its role.

Subscribers: arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67077

llvm-svn: 371041