Make PDBFile store an msf::Layout.

Make PDBFile store an msf::Layout.

Previously it was storing all the fields of an msf::Layout as
separate members.  This is a trivial cleanup to make it store
an msf::Layout directly.  This makes the code more readable
since it becomes clear which fields of PDBFile are actually the
msf specific layout information in a sea of other bookkeeping
fields.

llvm-svn: 276460