llvm-symbolizer: Move out of CLANG_EXTRAS, into CLANG

[kevans]

Summary

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.