[flang] Remove use of std::set::merge

Some versions of clang that we are building with don't have
std::set::merge, even though it is part of C++17. Work around
that by using std::set::insert until we can count on merge being
available everywhere.

Original-commit: flang-compiler/f18@886ccc37fbff5df00717ac728e2aba240d0a314c
Reviewed-on: https://github.com/flang-compiler/f18/pull/1014