Fix version check for LLVM < 6.0

[felixbuenemann]

This is a minor fixup for the LLVM 6.0 support quick fix (#6380) that changes the versions check for "below 6.0" (<= 5.9) to <= 5.0 since there wont be any new 5.x versions after 5.0.

[RX14]

Not sure this is worth merging, it works fine as-is.

[ilovezfs]

http://blog.llvm.org/2016/12/llvms-new-versioning-scheme.html

Patch releases will increment the patch component as before (producing versions like 5.0.1), and the minor component will stay at zero since no minor releases will be made.

[RX14]

This is either an incorrect PR or a purely-aesthetic change, depending on the implementation of LLVM_VERSION_LE. I can't be bothered to check which one it is, because it doesn't matter. I vote close.

[ilovezfs]

Why not change the 9 to a prettier number like 42?

[ysbaddaden]

Let's fixup. It fixes an imaginary version number, to one that does exist.

[felixbuenemann]

This is either an incorrect PR or a purely-aesthetic change, depending on the implementation of LLVM_VERSION_LE.

I can assure you it is not incorrect, the previous code was.

The LLVM_VERSION_LE macro only checks the major and minor version of LLVM. After LLVM 5.0 there is 6.0 and the next version will be 7.0. There are patch releases like 5.0.1/5.0.2, but there will never be a 5.1 release.

The only reason I initially used 5.9 as a check, is because I wanted to write "lower than 6.0" and then noticed the macro was lower-than-or-equal so I change it from 6.0 to 5.9, which technically works, but is confusing to the reader of the code.

[sdogruyol]

Thank you @felixbuenemann 👍