[libcxxabi] Use ASan interface header for declaration. NFC

This was changed from using the header to using a forward declaration in
c4600ccf, since older versions of the header didn't declare the
function. At this point, it's been declared for ~3.5 years, and it
should be pretty safe to assume that we can rely on the ASan interface
header to provide a declaration instead of needing to write our own.

Reviewed By: #libc_abi, ldionne

Differential Revision: https://reviews.llvm.org/D103003