Document -std= values for different languages

Summary:
After a remark on a FreeBSD mailing list that the clang man page did
not have any list of possible values for the `-std=` flag, I have now
attempted to exhaustively list those, for each available language.

I also documented the default standard for each language, if there was
more than one choice.

Reviewers: rsmith, dexonsmith, sylvestre.ledru, mgorny

Reviewed By: rsmith

Subscribers: fhahn, emaste, cfe-commits, krytarowski

Differential Revision: https://reviews.llvm.org/D45406

llvm-svn: 329827