[Sema] Fix PR30346: relax __builtin_object_size checks.

[Sema] Fix PR30346: relax __builtin_object_size checks.

This patch makes us act more conservatively when trying to determine
the objectsize for an array at the end of an object. This is in
response to code like the following:

```
struct sockaddr {
  /* snip */
  char sa_data[14];
};

void foo(const char *s) {
  size_t slen = strlen(s) + 1;
  size_t added_len = slen <= 14 ? 0 : slen - 14;
  struct sockaddr *sa = malloc(sizeof(struct sockaddr) + added_len);
  strcpy(sa->sa_data, s);
  // ...
}
```

`__builtin_object_size(sa->sa_data, 1)` would return 14, when there
could be more than 14 bytes at `sa->sa_data`.

Code like this is apparently not uncommon. FreeBSD's manual even
explicitly mentions this pattern:
https://www.freebsd.org/doc/en/books/developers-handbook/sockets-essential-functions.html
(section 7.5.1.1.2).

In light of this, we now just give up on any array at the end of an
object if we can't find the object's initial allocation.

I lack numbers for how much more conservative we actually become as a
result of this change, so I chose the fix that would make us as
compatible with GCC as possible. If we want to be more aggressive, I'm
happy to consider some kind of whitelist or something instead.

llvm-svn: 281277