Switch from clang 3.6 to new clang repository (with 3.8).

This reverts commit f7dbab16.

Bug: 23396112

Switch from "-fsanitize-undefined-trap-on-error" to
"-fsanitize-trap=all". The former ends up accidentally leaving
unresolved calls to __ubsan* helper functions in the object file with
clang 3.8. The latter is used when we don't include address sanitizer,
and replaces any misbehavior with a direct call to abort().