llvm-c: Don't assert in LLVMTargetMachineEmitToFile on nonexistent file

Error handling code for raw_fd_ostream constructor is present, but
never used, because formatted_raw_ostream will always assert on closed
fd's before.

Patch by Peter Zotov

Differential Revision: http://llvm-reviews.chandlerc.com/D1909

llvm-svn: 192881