Fix excess warnings from -Wtype-limits with location wrappers (PR c++/88680)

Fix excess warnings from -Wtype-limits with location wrappers (PR c++/88680)

PR c++/88680 reports excess warnings from -Wtype-limits after the C++
FE's use of location wrappers was extended in r267272 for cases such as:

  const unsigned n = 8;
  static_assert (n >= 0 && n % 2 == 0, "");

t.C:3:18: warning: comparison of unsigned expression >= 0 is always true
  [-Wtype-limits]
    3 | static_assert (n >= 0 && n % 2 == 0, "");
      |                ~~^~~~

The root cause is that the location wrapper around "n" breaks the
suppression of the warning for the "if OP0 is a constant that is >= 0"
case.

This patch fixes it by calling fold_for_warn on OP0, extracting the
constant.

gcc/c-family/ChangeLog:
	PR c++/88680
	* c-common.c (shorten_compare): Call fold_for_warn on op0 when
	implementing -Wtype-limits.

gcc/testsuite/ChangeLog:
	PR c++/88680
	* g++.dg/wrappers/pr88680.C: New test.

From-SVN: r268961