swift234

[serialization] Introduce ModuleID, with codes for special modules.

ModuleID is compatible with IdentifierID, but uses 0 to mean “the builtin module”
and 1 to mean “the current module”. Anything else is a top-level module name,
as an identifier. As an implementation detail, 1 is now never a valid IdentifierID.
(0 remains “the empty string”.)

Using this, simplify the encoding of the owner of a conformance.

Swift SVN r9944