Don't override __attribute__((no_stack_protector)) by inlining (PR52886)

Since 26c6a3e7, LLVM's inliner will "upgrade" the caller's stack protector
attribute based on the callee. This lead to surprising results with Clang's
no_stack_protector attribute added in 4fbf84c1 (D46300). Consider the
following code compiled with clang -fstack-protector-strong -Os
(https://godbolt.org/z/7s3rW7a1q).

  extern void h(int* p);

  inline __attribute__((always_inline)) int g() {
    return 0;
  }

  int __attribute__((__no_stack_protector__)) f() {
    int a[1];
    h(a);
    return g();
  }

LLVM will inline g() into f(), and f() would get a stack protector, against the
users explicit wishes, potentially breaking the program e.g. if h() changes the
value of the stack cookie. That's a miscompile.

More recently, bc044a88 (D91816) addressed this problem by preventing
inlining when the stack protector is disabled in the caller and enabled in the
callee or vice versa. However, the problem remained if the callee is marked
always_inline as in the example above. This affected users, see e.g.
http://crbug.com/1274129 and http://llvm.org/pr52886.

One way to fix this would be to prevent inlining also in the always_inline
case. Despite the name, always_inline does not guarantee inlining, so this
would be legal but potentially surprising to users.

However, I think the better fix is to not enable the stack protector in a
caller based on the callee. The motivation for the old behaviour is unclear, it
seems counter-intuitive, and causes real problems as we've seen.

This commit implements that fix, which means in the example above, g() gets
inlined into f() (also without always_inline), and f() is emitted without stack
protector. I think that matches most developers' expectations, and that's also
what GCC does.

Another effect of this change is that a no_stack_protector function can now be
inlined into a stack protected function, e.g. (https://godbolt.org/z/hafP6W856):

  extern void h(int* p);

  inline int __attribute__((__no_stack_protector__)) __attribute__((always_inline)) g() {
    return 0;
  }

  int f() {
    int a[1];
    h(a);
    return g();
  }

I think that's fine. Such code would be unusual since no_stack_protector is
normally applied to a program entry point which sets up the stack canary. And
even if such code exists, inlining doesn't change the semantics: there is still
no stack cookie setup/check around entry/exit of the g() code region, but there
may be in the surrounding context, as there was before inlining. This also
matches GCC.

See also the discussion at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94722

Differential revision: https://reviews.llvm.org/D116589