RFC: pass to warn on questionable uses of alloca().

[Aldy Hernandez <aldyher at gmail dot com>]

Hi folks!

I've been working on a plugin to warn on unbounded uses of alloca() to 
help find questionable uses in glibc and other libraries.  It occurred 
to me that the broader community could benefit from it, as it has found 
quite a few interesting cases. So, I've reimplemented it as an actual 
pass, lest it be lost in plugin la-la land and bit-rot.

Before I sink any more time cleaning it up, would this be something 
acceptable into the compiler?  It doesn't have anything glibc specific, 
except possibly the following idiom which I allow:

    if (gate_function (length))
        alloca(length);

...and the above is probably common enough that we should handle it.

The testcase has a lot of examples of what the pass handles.

Thoughts?

Aldy

p.s. The pass currently warns on all uses of VLAs.  I'm not completely 
sold on this idea, so perhaps we could remove it, or gate it with a flag.

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