swift234

[serialization] Serialize shared_external as shared.

We serialize shared_external linkage as shared since:

1. shared_external linkage is just a hack to tell the optimizer that a
shared function was deserialized.

2. We can not just serialize a declaration to a shared_external function
since shared_external functions still have linkonce_odr linkage at the LLVM
level. This means they must be defined not just declared.

Swift SVN r22562