[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
diff --git a/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp b/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
index b5875a0..481eb77 100644
--- a/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
+++ b/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
@@ -249,3 +249,8 @@
 std::unique_ptr<IPDBEnumTables> NativeSession::getEnumTables() const {
   return nullptr;
 }
+
+std::unique_ptr<IPDBEnumInjectedSources>
+NativeSession::getInjectedSources() const {
+  return nullptr;
+}