clover: Restore support for LLVM <= 3.9.

clover: Restore support for LLVM <= 3.9.

The commit 8e430ff8b060b4e8e922bae24b3c57837da6ea77 broke support for
LLVM 3.9 and older versions in Clover. This patch restores it and
refactors the support using Clover compatibility layer for LLVM.

v2: merged #ifdef blocks
v3: added support for LLVM 3.6-3.8
v4: add missing #ifdef around <memory>
v5: simplify using templates and lambda

Signed-off-by: Vedran Miletić <vedran@miletic.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98740
Tested-by[v4]: Pierre Moreau <pierre.morrow@free.fr>
Tested-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>