configure.ac: bump Clover LLVM requirement to 3.9

configure.ac: bump Clover LLVM requirement to 3.9

The only driver that utilises Clover already depends on LLVM 3.9.
Close to every supported distribution has said version.

Additionally libclc also requires LLVM 3.9.

With this in mind, we can safely bump the requirement.

There is a handful of dead code that we could remove, which will be
resolved with later commits.

Note: this drops the LLVM 3.6 build from the Travis build. LLVM 3.9 (and
later) are already covered in there.

https://lists.freedesktop.org/archives/mesa-dev/2017-September/170028.html

v2: Add reference to discussion thread (Eric), adjust libclc LLVM req.
(Jan).

Cc: Aaron Watry <awatry@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Acked-by: Vedran Miletić <vedran@miletic.net>
Acked-by: Jan Vesely <jan.vesely@rutgers.edu>
Acked-by: Francisco Jerez <currojerez@riseup.net>