COFF: add error() and warn() to Error.{cpp,h}

Summary: This copies over some functionality we have in ELF/Error.{cpp,h} and makes it available in COFF/Error.{cpp,h}

Reviewers: pcc, rafael, ruiu

Subscribers:

Differential Revision: https://reviews.llvm.org/D28692

llvm-svn: 292240