[PDB] Defer writing the build id until the rest of the PDB is written.

For now this is NFC, but this small refactor opens the door to
letting us embed a hash of the PDB in the build id field of the
PDB.

Differential Revision: https://reviews.llvm.org/D43913

llvm-svn: 326453
diff --git a/lld/COFF/PDB.cpp b/lld/COFF/PDB.cpp
index 60e69c1..de6aa64 100644
--- a/lld/COFF/PDB.cpp
+++ b/lld/COFF/PDB.cpp
@@ -1056,12 +1056,10 @@
 
   // Add an Info stream.
   auto &InfoBuilder = Builder.getInfoBuilder();
-  InfoBuilder.setAge(BuildId.PDB70.Age);
-
   GUID uuid;
   memcpy(&uuid, &BuildId.PDB70.Signature, sizeof(uuid));
+  InfoBuilder.setAge(BuildId.PDB70.Age);
   InfoBuilder.setGuid(uuid);
-  InfoBuilder.setSignature(time(nullptr));
   InfoBuilder.setVersion(pdb::PdbRaw_ImplVer::PdbImplVC70);
 
   // Add an empty DBI stream.