swift234

[serialization] Force derived top-level decls to be serialized.

Previously, serialization of a single source file only includes decls that
are within the SourceFile context. However, new top-level decls can be
added in order to derive protocol conformances; these decls need to be
serialized /somewhere/. Add the concept of decls "forced" to be serialized
along with the decls within the primary SourceFile context, and then mark
all derived top-level decls as "forced" if they come from a decl in the
primary source file.

Possibly a fix for <rdar://problem/16254101>, which crashes nearby.

Swift SVN r14817