Update llvm and clang to 3.5.

Needs to be merged with cms-sw/cmssw#5403.
Merge pull request #1044 from degano/clang_35_72X

Update llvm and clang to 3.5.
Merge remote branch 'origin/IB/CMSSW_7_2_X/stable' into IB/CMSSW_7_0_X/stable