[PECOFF] Implementation of ObjectFilePECOFF:: GetUUID()

Summary:
Provide an implementation of GetUUID() for remote debugging scenarios.

Return a PDB's GUID (or PDB70's Signature) as the UUID.

Reviewers: amccarth, labath

Reviewed By: labath

Subscribers: amccarth, clayborg, Hui, labath, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D56229

llvm-svn: 359528