[PATCH] Use -std=c++11 when no arguments are provided.

[Edwin Vane]

  I agree. If `-p` is used, even incorrectly, we shouldn't do anything with adding `-std=c++11` at all. If `--` is used explicitly it should take precedence  and we definitely do not add `-std=c++11` which is already happening. If `-p` is used, we know a value is required but if it's the empty string this should be an error.

http://llvm-reviews.chandlerc.com/D1337