Summary:
Add a new macro to track if folly was compiled with ASAN enabled.  This
is in addition to the existing FOLLY_SANITIZE_ADDRESS macro, which
reports if the current .cpp file is being compiled with ASAN enabled.

In particular, these macros differ in open source builds if folly was
compiled without ASAN enabled, but some of its headers are being
included from downstream libraries that are being built with ASAN
enabled.

Previously several headers would define structures differently based on
`FOLLY_SANITIZE_ADDRESS`, which meant that different .cpp files would see
different definitions of these structures.  `FOLLY_ASAN_ENABLED` is generally
the correct macro to use for situations like this.

This diff fixes F14Table.h to use FOLLY_ASAN_ENABLED, but several other
headers probably should also be updated in the future.

I did not change the behavior of FOLLY_SANITIZE_ADDRESS, since it appears to
be used by several projects outside of folly, and they expect it to report if
their code is being compiled with ASAN enabled.

At the moment the open source build does not support compiling with ASAN,
so FOLLY_ASAN_ENABLED is always disabled in open source builds.

Reviewed By: nbronson

Differential Revision: D10401271

fbshipit-source-id: 7e028737b42eecae93f06538dcf76dd64bd32e6a