lld-link: For nonexisting inputs, omit follow-on diagnostics

For lld-link missing.obj, lld-link currently prints:

  lld-link: error: could not open foo.obj: No such file or directory
  lld-link: warning: /machine is not specified. x64 is assumed
  lld-link: error: subsystem must be defined

The 2nd and 3rd diagnostics are consequences of the input not existing and are
not interesting. If input files are missing, the best thing we can do is point
that out and then return.

Differential Revision: https://reviews.llvm.org/D51981

llvm-svn: 342158