Specifically:
* disable BF16 when FP16 is disabled, since the former includes the
  latter;
* disable both when the toolkit version is lower than 12.2, since 12.2
  is when both types got the host versions of a lot of functions we need
  to make useful heterogeneous things with them;
* disable both in host-only TU, as there's no easy way I could find to
  detect the condition above. I've included an opt-in macro for
  asserting that the headers (if available) are from a sufficiently new
  CTK, will add that to docs in a later commit.

NVCC is spewing code that makes various versions of clang unhappy about
a deprecated implicit copy constructor of a lambda wrapper, so just work
around that by not using one.

Co-authored-by: Wesley Maxey <71408887+wmaxey@users.noreply.github.com>