[PATCH] warn on returning alloca and VLA (PR 71924, 90549)

[Martin Sebor]

-Wreturn-local-addr detects a subset of instances of returning
the address of a local object from a function but the warning
doesn't try to handle alloca or VLAs, or some non-trivial cases
of ordinary automatic variables[1].

The attached patch extends the implementation of the warning to
detect those.  It still doesn't detect instances where the address
is the result of a built-in such strcpy[2].

Tested on x86_64-linux.

Martin

[1] For example, this is only diagnosed with the patch:

   void* f (int i)
   {
     struct S { int a[2]; } s[2];
     return &s->a[i];
   }

[2] The following is not diagnosed even with the patch:

   void sink (void*);

   void* f (int i)
   {
     char a[6];
     char *p = __builtin_strcpy (a, "123");
     sink (p);
     return p;
   }

I would expect detecting to be possible and useful.  Maybe as
a follow-up.
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