Raising the minimum required LLVM version to 3.9

[Johan]

Hi all,
   Martin opened a PR that removes support for LLVM below 3.9.
https://github.com/ldc-developers/ldc/pull/2872

I'm in favor of it. LLVM 3.9 is 2 years old, and it's a burden to 
support so many LLVM versions.

Are there any good reasons to keep supporting LLVM < 3.9?

-Johan