[clang] Introduce -fstrict-flex-arrays=<n> for stricter handling of flexible arrays

Some code [0] consider that trailing arrays are flexible, whatever their size.
Support for these legacy code has been introduced in
f8f63249 but it prevents evaluation of
__builtin_object_size and __builtin_dynamic_object_size in some legit cases.

Introduce -fstrict-flex-arrays=<n> to have stricter conformance when it is
desirable.

n = 0: current behavior, any trailing array member is a flexible array. The default.
n = 1: any trailing array member of undefined, 0 or 1 size is a flexible array member
n = 2: any trailing array member of undefined or 0 size is a flexible array member
n = 3: any trailing array member of undefined size is a flexible array member (strict c99 conformance)

Similar patch for gcc discuss here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836

[0] https://docs.freebsd.org/en/books/developers-handbook/sockets/#sockets-essential-functions