Add an empty IPI stream.

With this, "llvm-pdbdump yaml -ipi-stream" prints out an IPI stream.
Previously it crashed because it can't handle the case where IPI
stream doesn't exist.

llvm-svn: 283392
diff --git a/lld/COFF/PDB.cpp b/lld/COFF/PDB.cpp
index 5d10395..2b9a39f 100644
--- a/lld/COFF/PDB.cpp
+++ b/lld/COFF/PDB.cpp
@@ -38,6 +38,8 @@
   ExitOnErr(Builder.getMsfBuilder().addStream(1));
   ExitOnErr(Builder.getMsfBuilder().addStream(1));
   ExitOnErr(Builder.getMsfBuilder().addStream(1));
+  ExitOnErr(Builder.getMsfBuilder().addStream(1));
+  ExitOnErr(Builder.getMsfBuilder().addStream(1));
 
   // Add an Info stream.
   auto &InfoBuilder = Builder.getInfoBuilder();
@@ -55,6 +57,10 @@
   auto &TpiBuilder = Builder.getTpiBuilder();
   TpiBuilder.setVersionHeader(pdb::PdbTpiV80);
 
+  // Add an empty IPI stream.
+  auto &IpiBuilder = Builder.getIpiBuilder();
+  IpiBuilder.setVersionHeader(pdb::PdbTpiV80);
+
   // Write to a file.
   ExitOnErr(Builder.commit(Path));
 }