Merge #1678 #1682 #1687

1678: Cleanup unused parameters r=CohenArthur a=philberty

This starts removing some of the unused parameters within the codebase

1682: const evaluator: Simplify get_nth_callarg r=CohenArthur a=CohenArthur

1687: Repair 'gcc/rust/lang.opt' comment r=CohenArthur a=tschwinge

... lost in #1527 commit 138a6260124740208b8f3aff2e38617f43b05fe8
"rust: Add -frust-compile-until option".

Co-authored-by: Philip Herron <herron.philip@googlemail.com>
Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>
Co-authored-by: Thomas Schwinge <thomas@codesourcery.com>

diff --cc gcc/rust/backend/rust-constexpr.cc
Simple merge

diff --cc gcc/rust/lang.opt
Simple merge