Optionally create full debuginfo for llvm-related executables

Commit de6feefdb7cfd limited the amount of debuginfo generated for clang
and other llvm-related executables. This was done to save disk space and
memory during building, but it makes debugging any of these executables
much harder.

Add a new src.conf(5) setting, WITH_LLVM_FULL_DEBUGINFO, to generate
full debuginfo instead. This is off by default, but could for example be
enabled for release builds or snapshots, so llvm executables are easier
to debug.

Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43839