llvm: add 3.4, urge for versioned usage

The 3.4 code was tested preliminary in x-updates,
described by 2e4eab1228.

Updates to llvm break builds of dependent packages (in all cases I've seen),
and often upstream isn't too fast in porting to the newest version.
Consequently, it seems better to keep more versions (two ATM),
both in one file to share eventual changes.
Also, using versioned llvm_* attributes is proposed because of this.