We actually do not need to pre-append the USR of the extended nominal;
a short mangling-incompatible prefix will sufficiently do the trick.
Suggested by @jrose-apple.
We did not serialize them because getting USR for extensions is tricky (USRs are
usually for value decls). This commit starts to make up an USR for an extension by combining
the extended nominal's USR with the USR of the first value member of the extension. We use
this made-up USR to associate doc comments when (de)serializing them.
This commit changes the Swift mangler from a utility that writes tokens into a
stream into a name-builder that has two phases: "building a name", and "ready".
This clear separation is needed for the implementation of the compression layer.
Users of the mangler can continue to build the name using the mangleXXX methods,
but to access the results the users of the mangler need to call the finalize()
method. This method can write the result into a stream, like before, or return
an std::string.
Change all the existing addressors to the unsafe variant.
Update the addressor mangling to include the variant.
The addressor and mutable-addressor may be any of the
variants, independent of the choice for the other.
SILGen and code synthesis for the new variants is still
untested.
Swift SVN r24387
