[codeview] respect signedness of APSInts when printing to YAML

Summary:
This fixes a bug where we always treat APSInts in Codeview as
signed when writing them to YAML. One symptom of this problem is that
llvm-pdbdump raw would show Enumerator Values that differ between the
original PDB and a PDB that has been round-tripped through YAML.

Reviewers: zturner

Reviewed By: zturner

Subscribers: llvm-commits, fhahn

Differential Revision: https://reviews.llvm.org/D34013

llvm-svn: 305965