Add support for unittest inputs.

Add support for unittest inputs.

Occasionally it is useful to have unittest which take inputs.
While we normally try to have this test be more of a lit test
we occasionally don't have tools that can exercise the code
in the right way to test certain things.  LLDB has been using
this style of unit test for a while, particularly with regards
to how it tests core dump and minidump file parsing.  Recently
i needed this as well for the case where we want to test that
some of the PDB reading code works correctly.  It needs to
exercise the code in a way that is not covered by any dumper
and would be impractical to implement in one of the dumpers,
but requires a valid PDB file.  Since this is now needed by
more than one project, it makes sense to have this be a
generally supported thing that unit tests can do, and we just
encourage people to use this sparingly.

Differential Revision: https://reviews.llvm.org/D51561

llvm-svn: 341502