[clang][auto-init] Provide __builtin_alloca*_uninitialized variants

When `-ftrivial-auto-var-init=` is enabled, allocas unconditionally
receive auto-initialization since [1].

In certain cases, it turns out, this is causing problems. For example,
when using alloca to add a random stack offset, as the Linux kernel does
on syscall entry [2]. In this case, none of the alloca'd stack memory is
ever used, and initializing it should be controllable; furthermore, it
is not always possible to safely call memset (see [2]).

Introduce `__builtin_alloca_uninitialized()` (and
`__builtin_alloca_with_align_uninitialized`), which never performs
initialization when `-ftrivial-auto-var-init=` is enabled.

[1] https://reviews.llvm.org/D60548
[2] https://lkml.kernel.org/r/YbHTKUjEejZCLyhX@elver.google.com

Reviewed By: glider

Differential Revision: https://reviews.llvm.org/D115440