[PDB] Support dumping injected sources via the DIA reader.

Injected sources are basically a way to add actual source file content
to your PDB. Presumably you could use this for shipping your source code
with your debug information, but in practice I can only find this being
used for embedding natvis files inside of PDBs.

In order to effectively test LLVM's natvis file injection, we need a way
to dump the injected sources of a PDB in a way that is authoritative
(i.e. based on Microsoft's understanding of the PDB format, and not
LLVM's). To this end, I've added support for dumping injected sources
via DIA. I made a PDB file that used the /natvis option to generate a
test case.

Differential Revision: https://reviews.llvm.org/D44405

llvm-svn: 327428