Refactor DragonFly BSD toolchain driver.

[rimvydas]

Summary

Make it more similar to FreeBSD one to reduce differences.
In preparations for later submissions.

While there, add more handling of flags (including OpenMP https://reviews.llvm.org/D35129).

[mgorny]

lib/Driver/ToolChains/DragonFly.cpp

118

Can't crt1 be non-null only if !Args.hasArg(options::OPT_shared here? i.e. is there a reason to do it like this instead of just pushing it inside the above if?

123

This default will never be used.

185

Inconsistency here: above you used helper variable, here you duplicate the whole line.

[rimvydas]

lib/Driver/ToolChains/DragonFly.cpp

118

The crt1 can only be used if !Args.hasArg(options::OPT_shared, both declaration and CmdArgs.push_back could be moved inside. The only reason for this adjustment is to reduce stylistic differences against lib/Driver/ToolChains/FreeBSD.cpp driver layout.

123

Correct. Could be changed in both FreeBSD.cpp and DragonFly.cpp.

185

Same as two previous. Only to match lib/Driver/ToolChains/FreeBSD.cpp layout.