swift234

[serialization] Rework cross-references to be more general / more robust.

Previously, cross-references just carried a chain of identifiers and a
top-level module, plus a type to validate against, a generic parameter index,
or an operator fixity. However, referencing "the first generic parameter
of the prefix function ++ that takes a ForwardIndex" requires /all three/
of these filters to unambiguously select the right declaration.

Now, cross-references consist of a chain of trailing records, one for each
link in the path. There are (currently) five kinds of links:

Type: a declaration that cannot have overloads
Value: a declaration that can have overloads (filtered by type)
Extension: filter to decls within extensions on another module
Operator:
- as the first path piece, an operator declaration
- as a later path piece, a fixity filter for operator functions
Generic Param: an indexed generic parameter of the previous result

This should allow us to uniquely cross-reference any Swift declaration we
need to.

Swift SVN r11399