Previously, the Lexer kept a single flag whether we’re lexing Swift or SIL. Instead, keep track if we’re parsing Swift, SIL, or a Swiftinterface file. .swiftinterface files allow $-prefixed identifiers anywhere.
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.
