[clang] Correct sanitizer behavior in union FAMs

Clang doesn't have the same behavior as GCC does with union flexible
array members. (Technically, union FAMs are probably not acceptable in
C99 and are an extension of GCC and Clang.)

Both Clang and GCC treat *all* arrays at the end of a structure as FAMs.
GCC does the same with unions. Clang does it for some arrays in unions
(incomplete, '0', and '1'), but not for all. Instead of having this
half-supported feature, sync Clang's behavior with GCC's.

Reviewed By: kees

Differential Revision: https://reviews.llvm.org/D135727