Rollup merge of #89753 - jkugelman:must-use-from_value-conversions, r=joshtriplett

Add #[must_use] to from_value conversions

I added two methods to the list myself. Clippy did not flag them because they take `mut` args, but neither modifies their argument.

```rust
core::str           const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str;
std::ffi::CString   unsafe fn from_raw(ptr: *mut c_char) -> CString;
```

I put a custom note on `from_raw`:

```rust
#[must_use = "call `drop(from_raw(ptr))` if you intend to drop the `CString`"]
pub unsafe fn from_raw(ptr: *mut c_char) -> CString {
```

Parent issue: #89692

r? ``@joshtriplett``

library/alloc/src/str.rs

@@ -595,6 +595,7 @@ impl str {
 /// assert_eq!("☺", &*smile);
 /// ```
 #[stable(feature = "str_box_extras", since = "1.20.0")]
+#[must_use]
 #[inline]
 pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
     unsafe { Box::from_raw(Box::into_raw(v) as *mut str) }

library/alloc/src/string.rs

@@ -764,6 +764,7 @@ impl String {
     /// assert_eq!("💖", sparkle_heart);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
         String { vec: bytes }