Store TypeUnits in a SmallVector<DWARFUnitSection> instead of a single DWARFUnitSection.

Summary:
There will be multiple TypeUnits in an unlinked object that will be extracted
from different sections. Now that we have DWARFUnitSection that is supposed
to represent an input section, we need a DWARFUnitSection<TypeUnit> per
input .debug_types section.

Once this is done, the interface is homogenous and we can move the Section
parsing code into DWARFUnitSection.

Reviewers: samsonov, dblaikie

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5482