tools build: Fix test-clang.cpp with Clang 8+

LLVM rL344140 (included in Clang 8+) moved VFS from Clang to LLVM, so
paths to its include files have changed.

This broke the Clang test in tools/build - let's fix it.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Denis Pronin <dannftk@yandex.ru>
Cc: Dennis Schridde <devurandom@gmx.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Naohiro Aota <naota@elisp.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: clang-built-linux@googlegroups.com
Link: http://lore.kernel.org/lkml/20191228171314.946469-1-mail@maciej.szmigiero.name
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>