c++: Move new test to 'opt' sub-directory

gcc/testsuite/ChangeLog:

	* g++.dg/pr110879.C: Moved to...
	* g++.dg/opt/pr110879.C: ...here.

diff --git a/gcc/testsuite/g++.dg/pr110879.C b/gcc/testsuite/g++.dg/opt/pr110879.C
similarity index 100%
rename from gcc/testsuite/g++.dg/pr110879.C
rename to gcc/testsuite/g++.dg/opt/pr110879.C