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 set TREE_NO_WARNING
selectively on those COMPOUND_EXPRs that need them.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check with no new failures.  Ok for mainline?

2022-07-16  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR c/106264
        * builtins.cc (fold_builtin_frexp): Set TREE_NO_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.


Thanks in advance,
Roger
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