-Wmissing-attributes: check that we avoid duplicates and false positives

-Wmissing-attributes: check that we avoid duplicates and false positives

The initial patch for PR 81824 fixed various possibilities of
-Wmissing-attributes reporting duplicates and false positives.  The
test that avoided them was a little obscure, though, so this patch
rewrites it into a more self-evident form.

The patch also adds a testcase that already passed, but that
explicitly covers some of the possibilities of reporting duplicates
and false positives that preexisting tests did not cover.

for  gcc/ChangeLog

	PR middle-end/81824
	* attribs.c (decls_mismatched_attributes): Simplify the logic
	that avoids duplicates and false positives.

for  gcc/testsuite/ChangeLog

	PR middle-end/81824
	* g++.dg/Wmissing-attributes-1.C: New.  Some of its fragments
	are from Martin Sebor.

From-SVN: r273563