[PATCH] implement -Winfinite-recursion [PR88232]

[Martin Sebor]

The attached patch adds support to the middle end for detecting
infinitely recursive calls.  The warning is controlled by the new
-Winfinite-recursion option.  The option name is the same as
Clang's.

I scheduled the warning pass to run after early inlining to
detect mutually recursive calls but before tail recursion which
turns some recursive calls into infinite loops and so makes
the two indistinguishable.

The warning detects a superset of problems detected by Clang
(based on its tests).  It detects the problem in PR88232
(the feature request) as well as the one in PR 87742,
an unrelated problem report that was root-caused to bug due
to infinite recursion.

This initial version doesn't attempt to deal with superimposed
symbols, so those might trigger false positives.  I'm not sure
that's something to worry about.

The tests are very light, but those for the exceptional cases
are exceedingly superficial, so it's possible those might harbor
some false positives and negatives.

Tested on x86_64-linux.

Martin

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