[PATCH] issue -Walloca even when alloca is a system header macro [PR94004]

[Martin Sebor]

A 2017 change that propagated call location to GIMPLE statements
had the unexpected (though not incorrect) effect of setting
the location on calls to __builtin_alloca.  When such calls are
the result of the expansion of the alloca macro defined in
a system header like <alloca.h>, the change prevents -Walloca and 
-Walloca-larger-than
warnings from being issued unless -Wsystem-headers is also used,
effectively defeating the point of the first two warnings in most
code.

The attached patch changes -Walloca to use the location of the point
of the macro expansion rather than that of its definition, restoring
the intended behavior.

In addition, to make the warnings (mainly -Wvla) in inlined code
easier to debug, the patch also arranges for the warnings to include
their inlining context.

Tested on x86_64-linux.

Martin
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