...finally breaking the dependency of Parse on Sema.
There are still some unfortunate dependencies here -- Xi's working on
getting /AST/ not dependent on Sema -- but this is a step forward.
It is a little strange that parseIntoSourceFile is in ParseSIL, and
therefore that that's still a dependency for anyone trying to, well,
parse. However, nearly all clients that parse want to type-check as
well, and that requires Sema, Serialization, and the ClangImporter...
and Serialization and SIL currently require each other as well
(another circular dependency). So it's not actively causing us trouble
right now.
