Build fail fix for ISPC with LLVM 3.6 (current trunk)

Build fails because of the new interface for 'DIBuilder::insertDeclare' and 'DIBuilder::createGlobalVariable' in LLVM 3.6 (ids: 076fd5dfc1f0600183bbc7db974dc7b39086136d and bc88cfc3512326d6c8f8dbf3c15d880c0e21feb0 correspondingly)