ldc depends on phobos

[Joakim]

I tried building ldc without linking against phobos and got a 
handful of link errors.  Has it been decided to selectively use 
phobos in the ldc compiler?

For example, pruneCache is implemented in D and called from 
codegenModules:

https://github.com/ldc-developers/ldc/blob/master/driver/main.cpp#L1122
https://github.com/ldc-developers/ldc/blob/master/driver/cache.cpp#L481
https://github.com/ldc-developers/ldc/blob/master/driver/cache_pruning.d#L28

The last file with the D implementation uses imports from phobos, 
it was added before the 1.1 release:

https://github.com/ldc-developers/ldc/pull/1753

There's another in the frontend:

https://github.com/ldc-developers/ldc/blob/master/ddmd/mtype.d#L2848

And finally, some functions from ddmd.builtin seem to call some 
std.math functions.