Reapply "ADT: Fix reference invalidation in SmallVector::push_back and single-element insert"

This reverts commit 56d1ffb9, reapplying
9abac603, removing insert_one_maybe_copy
and using a helper called forward_value_param instead. This avoids use
of `std::is_same` (or any SFINAE), so I'm hoping it's more portable and
MSVC will be happier.

Original commit message follows:

For small enough, trivially copyable `T`, take the argument by value in
`SmallVector::push_back` and copy it when forwarding to
`SmallVector::insert_one_impl`. Otherwise, when growing, update the
argument appropriately.

Differential Revision: https://reviews.llvm.org/D93779