llvm: add a build knob for enabling assertions

For head/, this will remain eternally default-on to maintain the status quo.
For stable/ branches, it should be flipped to default-off to maintain the
status quo.

There's value in being able to flip it one way or the other easily on head
or stable branches, whether you want to gain some performance back on head/
(for machines there's little chance you'll actually hit an assertion) or
potentially diagnose a problem with the version of llvm on an older branch.

Currently, stable branches get the CFLAGS+= -ndebug line uncommented; going
forward, they will instead have the default of LLVM_ASSERTIONS flipped.

Reviewed by: dim, emaste, re (gjb)
MFC after: 1 week
MFC note: flip the default of LLVM_ASSERTIONS
Differential Revision: https://reviews.freebsd.org/D24264