Consider calls from implict host device functions as valid in SemaCUDA.

In SemaCUDA all implicit functions were considered host device, this led to
errors such as the following code snippet failing to compile:

struct Copyable {
  const Copyable& operator=(const Copyable& x) { return *this; }
};

struct Simple {
  Copyable b;
};

void foo() {
  Simple a, b;

  a = b;
}

Above the implicit copy assignment operator was inferred as host device but
there was only a host assignment copy defined which is an error in device
compilation mode.

Differential Revision: http://reviews.llvm.org/D6565