Rollup merge of #61134 - nvzqz:reverse_bits-must_use, r=varkor

Annotate each `reverse_bits` with `#[must_use]`

Because the name sounds like an in-place mutation like `[T]::reverse(&mut self)`, it may be confused for one.

This change was requested at https://github.com/rust-lang/rust/issues/48763#issuecomment-493743741.

src/libcore/num/mod.rs

@@ -473,6 +473,7 @@ assert_eq!(m, ", $reversed, ");
             #[unstable(feature = "reverse_bits", issue = "48763")]
             #[rustc_const_unstable(feature = "const_int_conversion")]
             #[inline]
+            #[must_use]
             pub const fn reverse_bits(self) -> Self {
                 (self as $UnsignedT).reverse_bits() as Self
             }
@@ -2522,6 +2523,7 @@ assert_eq!(m, ", $reversed, ");
 ```"),
             #[unstable(feature = "reverse_bits", issue = "48763")]
             #[inline]
+            #[must_use]
             pub const fn reverse_bits(self) -> Self {
                 intrinsics::bitreverse(self as $ActualT) as Self
             }

src/libcore/num/wrapping.rs

@@ -524,6 +524,7 @@ assert_eq!(n.trailing_zeros(), 3);
             /// ```
             #[unstable(feature = "reverse_bits", issue = "48763")]
             #[inline]
+            #[must_use]
             pub const fn reverse_bits(self) -> Self {
                 Wrapping(self.0.reverse_bits())
             }