Auto merge of #34178 - shepmaster:llvm-3.7-and-up, r=alexcrichton

Reflect supporting only LLVM 3.7+ in the LLVM wrappers

Based on 12abddb06b681f5c1cb389074b5a35d3e260698f, it appears we can drop support for these older LLVM versions. Hopefully, this will make it slightly easier to support the changes needed for LLVM 3.9.

r? @nagisa

/cc @brson