PR c/106264: Silence warnings from __builtin_modf et al.

PR c/106264: Silence warnings from __builtin_modf et al.

This middle-end patch resolves PR c/106264 which is a spurious warning
regression caused by the tree-level expansion of modf, frexp and remquo
producing "expression has no-effect" when the built-in function's result
is ignored.  When these built-ins were first expanded at tree-level,
fold_builtin_n would blindly set TREE_NO_WARNING for all built-ins. Now
that we're more discerning, we should precisely call suppress_warning
selectively on those COMPOUND_EXPRs that need them.

2022-07-19  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR c/106264
	* builtins.cc (fold_builtin_frexp): Call suppress_warning on
	COMPOUND_EXPR to silence spurious warning if result isn't used.
	(fold_builtin_modf): Likewise.
	(do_mpfr_remquo): Likewise.

gcc/testsuite/ChangeLog
	PR c/106264
	* gcc.dg/pr106264.c: New test case.