[DebugInfo][PDB] Don't write empty debug streams

[DebugInfo][PDB] Don't write empty debug streams

Before, empty debug streams were written as 8 bytes (4 bytes signature + 4 bytes for the GlobalRefs count).

With this patch, unused empty streams aren't emitted anymore. Modules now encode 65535 as an 'unused stream' value, by convention.
Also fix the * Linker * contrib section which wasn't correctly emitted previously.

Differential Revision: https://reviews.llvm.org/D59502

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356395 91177308-0d34-0410-b5e6-96231b3b80d8