travis: meson: use native files to override llvm-config

travis: meson: use native files to override llvm-config

This is the supported way to do this, and should be more robust and
reliable.

v2: [Emil]
 - enable backslash escapes
 - don't hardcode the path
 - pass the argument directly to meson

Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v1)
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>