Use tag types to generate attribute specific internals and avoid ODR violations

[DavisVaughan]

Closes #491
Closes #422 (because all this did was change the warning() signature to something that no longer conflicted with stop(), so it didn't fix the underlying problem)
This was the actual solution for #295

My inline documentation fully describes the problem, so I will regurgitate it here

// Tag types to force templated `struct closure` and `apply()` infrastructure shared
// across `struct function` and `struct noreturn_function` to generate different
// attribute specific `struct closure` and `apply()` variants.
//
// Consider:
//
// ```
// cpp11::stop("error: %s", message)
// cpp11::warning("warning: %s", message)
// ```
//
// These both end up constructing the exact same templated `struct closure` and `apply()`
// functions. The `args` for the underlying `Rf_errorcall()` and `Rf_warningcall()` are:
// - `R_NilValue`
// - `const char* fmt`
// - `const char* message`
//
// The only difference is that `cpp11::stop()` is marked as `[[noreturn]]` because the
// underlying `Rf_errorcall()` is also marked as `[[noreturn]]` /
// `__attribute__((noreturn))`.
//
// But this causes issues! Due to C++'s ODR (One Definition Rule), only 1 variant of
// `apply()` and `struct closure` can be created per template combination. If the
// `cpp11::stop()` variant is linked in first, then some compilers use the `[[noreturn]]`
// hint on `cpp11::stop()` and `operator()` of `noreturn_function` to assert that the
// `apply()` function also cannot return, and returning is deemed unreachable. So then
// when `cpp11::warning()` tries to return from its call to `apply()`, a crash occurs. We
// see this output under ASAN: `execution reached an unreachable program point`.
//
// We've seen this issue on macOS and Linux under clang (gcc does not seem to reproduce
// this). To reproduce, you must have `cpp11::stop()` and `cpp11::warning()` calls in
// different translation units / files and the file containing `cpp11::stop()` must be
// linked first. Putting it first alphabetically seems to be enough, which is why we have
// `template-1-stop.cpp` and `template-2-warn.cpp` in our tests, along with
// `test-template.R` to test this exact issue. You also need to compile with `-O0`,
// otherwise you'll just get a hang rather than a crash.
//
// Adding the tag into the template definition forces `safe[fn]()` and
// `safe.noreturn[fn]()` calls to generate different `apply()` variants, avoiding this
// issue.
//
// https://github.com/r-lib/cpp11/issues/491
// https://github.com/r-lib/cpp11/issues/295
struct return_tag {};
struct no_return_tag {};

[DavisVaughan]

Some additional prior issues / prs related to this

#85
#87

My theory is that these just didn't go far enough in making sure the internals were also attribute specific

[DavisVaughan]

@jimhester - @jennybc and I thought you'd enjoy the resolution to this gnarly issue that has caused cpp11::warning() to crash for unexplainable reasons over the years