[llvm-pdbdump] Dump MSF headers to YAML.

This is the simplest possible patch to get some kind of YAML
output.  All it dumps is the MSF header fields so that in
theory an empty MSF file could be reconstructed.

Reviewed By: ruiu, majnemer
Differential Revision: http://reviews.llvm.org/D20971

llvm-svn: 271939