lld-link: Write an empty "repro" debug directory entry if /Brepro is passed

If the coff timestamp is set to a hash, like lld-link does if /Brepro is
passed, the coff spec suggests that a IMAGE_DEBUG_TYPE_REPRO entry is in the
debug directory. This lets lld-link write such a section.
Fixes PR38429, see bug for details.

Differential Revision: https://reviews.llvm.org/D51652

llvm-svn: 341486