Create a frontend flag to disable CUDA cross-target call checks

For CUDA source, Sema checks that the targets of call expressions make sense
(e.g. a host function can't call a device function).

Adding a flag that lets us skip this check. Motivation: for source-to-source
translation tools that have to accept code that's not strictly kosher CUDA but
is still accepted by nvcc. The source-to-source translation tool can then fix
the code and leave calls that are semantically valid for the actual compilation
stage.

Differential Revision: http://reviews.llvm.org/D9036

llvm-svn: 235049