Use Backend_EmitMCNull for null codegen unit tests.

Using Backend_EmitLL attemps to create a file with an empty filename.
This is problematic in certain environments: an empty filename may be
illegal, or the default output path may not be writable (in the case
where an empty filename would otherwise have some non-failing
semantics). This patch switches to use Backend_EmitMCNull, which
allows CodeGen to run, but does not attempt to create or write an
output file.

Differential Revision: http://reviews.llvm.org/D17405

llvm-svn: 261252