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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261252 91177308-0d34-0410-b5e6-96231b3b80d8