Use #[inline(always)] on trivial UnsafeCell methods

UnsafeCell is the standard building block for shared mutable data
structures. UnsafeCell should add zero overhead compared to using raw
pointers directly.

Some reports suggest that debug builds, or even builds at opt-level 1,
may not always be inlining its methods. Mark the methods as
`#[inline(always)]`, since once inlined the methods should result in no
actual code other than field accesses.

library/core/src/cell.rs

@@ -1815,7 +1815,7 @@ impl<T> UnsafeCell<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
-    #[inline]
+    #[inline(always)]
     pub const fn new(value: T) -> UnsafeCell<T> {
         UnsafeCell { value }
     }
@@ -1831,7 +1831,7 @@ impl<T> UnsafeCell<T> {
     ///
     /// let five = uc.into_inner();
     /// ```
-    #[inline]
+    #[inline(always)]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
     pub const fn into_inner(self) -> T {
@@ -1856,7 +1856,7 @@ impl<T: ?Sized> UnsafeCell<T> {
     ///
     /// let five = uc.get();
     /// ```
-    #[inline]
+    #[inline(always)]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
     pub const fn get(&self) -> *mut T {
@@ -1881,7 +1881,7 @@ impl<T: ?Sized> UnsafeCell<T> {
     ///
     /// assert_eq!(*c.get_mut(), 6);
     /// ```
-    #[inline]
+    #[inline(always)]
     #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
     pub fn get_mut(&mut self) -> &mut T {
         &mut self.value
@@ -1914,7 +1914,7 @@ impl<T: ?Sized> UnsafeCell<T> {
     ///
     /// assert_eq!(uc.into_inner(), 5);
     /// ```
-    #[inline]
+    #[inline(always)]
     #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")]
     pub const fn raw_get(this: *const Self) -> *mut T {
         // We can just cast the pointer from `UnsafeCell<T>` to `T` because of