Add check-exit-code option, defaulting to true.

Add ProgramExitedNonzero argument to executeProgram(), and make it
tell its caller whether the program exited nonzero.

Move executeProgramWithCBE() out of line, to ExecutionDriver.cpp, and remove
its extra arguments which are always defaulted.  Make it turn off
check-exit-code if the program exits nonzero while generating a reference
output.

Make diffProgram() assume that any nonzero exit code is a failure, if
check-exit-code is turned on.

llvm-svn: 11325