When we deserialize a declaration from a module file, allocate extra

storage for the global declaration ID. Declarations that are parsed
(rather than deserialized) are unaffected, so the number of
declarations that pay this cost tends to be relatively small (since
relatively few declarations are ever deserialized).

This replaces a largish DenseMap within the AST reader. It's not
strictly a win in terms of memory use---not every declaration was
added to that DenseMap in the first place---but it's cleaner to have
this information available for every deserialized declaration, so that
future clients can rely on it.

llvm-svn: 147617