Add Lexer::IsHashbangAllowed, drop SourceManager::getHashbangBufferID (#18534)
Having this be a single buffer hardcoded in the SourceManager and set
by all clients is silly. SourceFiles with the 'Main' kind are allowed
to have hashbang lines (`#!`), other files are not. And anyone
manually setting up a Lexer can decide for themselves.
No intended behavioral change.