llvm-symbolizer: Move out of CLANG_EXTRAS, into CLANG

ASAN reports become a lot more useful with llvm-symbolizer in $PATH, and the
build is not much more time-consuming. The added benefit is that the
resulting reports will actually include symbol information; without, thread
trace information includes a bunch of addresses that immediately resolve to
an inline function in
^/contrib/compiler-rt/lib/sanitizer_common/sanitizer_common.h and take a
little more effort to examine.

Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20484