[Serialization] Preserve whether a raw value is explicit (#26324)
...which allows the AST printer to correctly choose whether to print
it, which means it can be printed in a module interface in a non-WMO
build, which is necessary for @objc enums to have a correct run-time
representation when clients use that interface.
rdar://problem/53469608
(cherry picked from commit 28b3d8e0a708b67128586da2423c373603dd38ac)
