Hide obsucre LLVM flags on versions that support it.

The idea is to hide any flags that an end user is unlikely
to need so that the -help output gets clearer. Some of the
flags are also downright confusing, as they seem similar
to unrelated D concepts. The hidden flags are still
available using -help-hidden.

Unfortunately, this is only possible on LLVM 3.3+.
Capitalize option decriptions.
More accurate description for "-vv".

Still not happy with it, but I feel it is still better
than just "very verbose".
Merge pull request #512 from klickverbot/help-cleanup

-help output cleanup