Avoid allocations in givens

One problem here was that convert on tuples calls convert_tuple even if
no conversion is necessary, and convert_tuple is a built-in that needs
a jl_tuple_t. A second problem was that the tuple elimination pass runs
after the inlining pass, so the constructor wasn't getting inlined.
Merge pull request #6233 from JuliaLang/sjk/alloc-givens

Avoid allocations in givens