Raising the minimum required LLVM version to 3.9

[Nicholas Wilson]

On Monday, 8 October 2018 at 19:02:33 UTC, Johan wrote:
> 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

I can't think of any. Sounds like a good idea.