Bug#586900: llvm-2.7: Please use the debian alternatives system

[Matthias Klose]

 > The debian alternatives system should be able to make this
 > relatively easy.

No. Using the alternatives system for potentially ABI incompatible versions does 
hide problems and make it intransparent which llvm version is actually used. 
The explicit version should be used.  If the llvm maintainers decide to move on 
the next llvm version, then the default versions should be changed to point to 
the new version.