Simplify file handling in dsymutil.

This moves the file handling out of DwarfLinker.cpp.

This fixes what is at least an oddity if not a bug. DwarfLinker.cpp
was using ToolOutputFile, which uses RemoveFileOnSignal. The issue is
that dsymutil.cpp uses that too. It is now clear from the interface
that only dsymutil.cpp is responsible for creating and deleting files.

llvm-svn: 318334