Add suggestion to .to_owned() used on Cow when borrowing

[Periodic1911]

Fixes #144792

Adds a suggestion when using .to_borrow() on Cow where .into_borrow() should be used, and makes a test that explicitly checks if it is shown.

[rustbot]

r? @davidtwco

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[rustbot]

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

[Periodic1911]

I'm not happy about detecting the use of .to_owned by way of string comparison, but since this check is in the MIR I don't know how to do it better. Constructive criticism would be greatly appreciated!

[Kivooeo]

Some test adjustments

We have a bit of a gap in our test coverage, so I'm trying to keep a close eye on new tests

[Kivooeo]

I'd personally move this test to suggestions/ or borrowck/ and about name... I think name is fine but we can do better right? So i'd suggest something like cow-into-owned-suggestion.rs

[Kivooeo]

It's better to use full links and special comment //! Regression test for: https://github.com/rust-lang/rust/issues/144792 just because it will be easy to follow with just copy past link

[Kivooeo]

It might be fine but I don't like it's env dependable, can we have more interesting test like below and with more coverage

// More robust - doesn't depend on environment
fn test_cow_suggestion() -> String {
    let os_string = std::ffi::OsString::from("test");
    os_string.to_string_lossy().to_owned()
}

// Test multiple Cow scenarios
fn test_cow_from_str() -> String {
    let s = "hello";
    let cow = std::borrow::Cow::from(s);
    cow.to_owned() // Should suggest into_owned()
}

// Test with different Cow types
fn test_cow_bytes() -> Vec<u8> {
    let bytes = b"hello";
    let cow = std::borrow::Cow::from(&bytes[..]);
    cow.to_owned() // Should suggest into_owned()
}