LLVM Support Range Bump (dropping LLVM 10 support)

[Matthew Pickering]

Hi all,

We are intending to bump the range of supported LLVM versions in the
9.6, release.

For 9.4 the range is 10-14 (inclusive)

For 9.6 we will support 11-15 (inclusive)

If dropping support for LLVM 10 is a concern for anyone please let us know.

Cheers,

Matt