Module references get indexed as a 'module' symbol; they get USRs similar to how clang would assign a USR for a module reference.
JIRA: https://bugs.swift.org/browse/SR-8677
Typically, users jump to type-specific interface from a member of that type, for
instance, a.getSomething(). To generate the interface, we need to report the USR
of the container type of "getSomething()", which is the USR for the type of a,
when cursor info is requested for this function call.
The mangled name of the type is identical to those for debugger. These
mangled names allow us to reconstruct the type from AST and generate interface
specifically for that type.
Related rdar://27306890
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.
