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

[Edwin Vane]

  The documentation should be updated to reflect this new behaviour. It should be explicitly spelled out instead of referring to other documentation. Should talk about the priority of input:

  # If `--` is used, the args after, if any, are used
  # If `-p` is used, that compilation database is used. If the compilation database can't be used for any reason, this is an error.
  # If no `-p` or `--` are used, a compilation database is searched for based on the path of the first source file. If one doesn't exist, or exists but can't be used for some reason, then `-std=c++11` is used as the only compilation option.

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