auto-init of __builtin_alloca

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

In certain cases, it turns out, this is causing problems [2], as it
causes significant overheads with no way to turn off that behaviour.

The more fundamental problem, that GCC devs pointed out to us [3], is
that an explicit __builtin_alloca() is not an "automatic variable",
which is also why GCC 12's behaviour of the same command-line option
diverges from Clang's behaviour.

As the command-line option's name suggestions, and also our
documentation [4] does point out that this is for "trivial automatic
stack variables".

From all that, the conclusion appears to be that initializing

__builtin_alloca is out-of-scope of ftrivial-auto-var-init.

Without entirely reverting __builtin_alloca initialization, the cleanest
option appears to be introducing yet another -f<option> to enable
auto-init of allocas.

Another alternative is to entirely leave alloca() initialization to libc
definitions of alloca():

  // strawman
  static inline void *alloca(size_t size)
    void *ret = alloca(size);
  #ifdef __ALLOCA_AUTO_INIT__
    memset(ret, 0, size);
    return ret;


  -- Marco