[FR]: Add MatchVariant helper for std::visit + Overload

[hocheung-chromium]

Describe the issue

Motivation

When using absl::Overload with std::visit, the syntax requires constructing an explicit Overload object:

std::visit(absl::Overload{ 
    [](int) { ... },
    [](std::string) { ... }
}, my_variant);

Adding a MatchVariant helper would simplify this to:

absl::MatchVariant(my_variant,
    [](int) { ... },
    [](std::string) { ... });

Proposed implementation

template <typename Variant, typename... T>
auto MatchVariant(const Variant& v, T... args) {
    return std::visit(Overload{args...}, v);
}

Steps to reproduce the problem

N/A

What version of Abseil are you using?

Latest version

What operating system and version are you using?

Debian testing

What compiler and version are you using?

clang

What build system are you using?

Ninja

Additional context

No response

[derekmauro]

@jbandela

FYI I'm asking around for feedback on this proposal.