[PDB] Add support for parsing Flags from PDB Stream.

[PDB] Add support for parsing Flags from PDB Stream.

This was discovered when running `llvm-pdbdump diff` against
two files, the second of which was generated by running the
first one through pdb2yaml and then yaml2pdb.

The second one was missing some bytes from the PDB Stream, and
tracking this down showed that at the end of the PDB Stream were
some additional bytes that we were ignoring.  Looking back
to the reference code, these seem to specify some additional
flags that indicate whether the PDB supports various optional
features.

This patch adds support for reading, writing, and round-tripping
these flags through YAML and the raw dumper, and updates the
tests accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297984 91177308-0d34-0410-b5e6-96231b3b80d8