CodeView: extract the OMF Directory Header

The DebugDirectory contains a pointer to the CodeView info structure which is a
derivative of the OMF debug directory.  The structure has evolved a bit over
time, and PDB 2.0 used a slightly different definition from PDB 7.0.  Both of
these are specific to CodeView and not COFF.  Reflect this by moving the
structure definitions into the DebugInfo/CodeView headers.  Define a generic
DebugInfo union type that can be used to pass around a reference to the
DebugInfo irrespective of the versioning.  NFC.

llvm-svn: 278075