[llvm-pdbdump] in yaml2pdb, generate default output filename if none given

Summary:
llvm-pdbdump yaml2pdb used to fail with a misleading error
message ("An I/O error occurred on the file system") if no output file
was specified. This change adds an assert to PDBFileBuilder to check
that an output file name is specified, and makes llvm-pdbdump generate
an output file name based on the input file name if no output file
name is explicitly specified.

Reviewers: amccarth, zturner

Reviewed By: zturner

Subscribers: fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D33296

llvm-svn: 303299