Create lib/DebugInfo/PDB.

Description

Create lib/DebugInfo/PDB.

This patch creates a platform-independent interface to a PDB reader.
There is currently no implementation of this interface, which will
be provided in future patches. This defines the basic object model
which any implementation must conform to.

Reviewed by: David Blaikie
Differential Revision: http://reviews.llvm.org/D7356

Details