PR c++/86586 - -fcompare-debug=-Wsign-compare.

PR c++/86586 - -fcompare-debug=-Wsign-compare.

This patch limits constexpr folding for -Wsign-compare to only cases that we
would warn for without considering constant values, avoiding the folding in
the testcase in question.

gcc/c-family/
	* c-warn.c (warn_for_sign_compare): Call fold_for_warn.
gcc/cp/
	* typeck.c (cp_build_binary_op): Don't fold for -Wsign-compare.

From-SVN: r270136