swift234

SILSerializer: handle SILWitnessTable.

We add two records in sil_block to specify a witness table record and a method
entry record. Out of the four entry types, only "Method" is handled in this
commit.

Two records are also added to sil_index_block to search for a specific witness
table given a unique identifier. The interface lookupWitnessTable is not
implemented yet.

Right now, we serialize a witness table only when sil-serialize-all is on and
deserialize all witness tables in the module when sil-link-all is on.

rdar://15722175


Swift SVN r13000