Make cow_is_borrowed methods const

Constify the following methods of `alloc::borrow::Cow`:
 - `is_borrowed`
 - `is_owned`

These methods are still unstable under `cow_is_borrowed`.
Possible because of rust-lang#49146 (Allow if and match in constants).

Tracking issue: rust-lang#65143

library/alloc/src/borrow.rs

@@ -217,7 +217,7 @@ impl<B: ?Sized + ToOwned> Cow<'_, B> {
     /// assert!(!bull.is_borrowed());
     /// ```
     #[unstable(feature = "cow_is_borrowed", issue = "65143")]
-    pub fn is_borrowed(&self) -> bool {
+    pub const fn is_borrowed(&self) -> bool {
         match *self {
             Borrowed(_) => true,
             Owned(_) => false,
@@ -239,7 +239,7 @@ impl<B: ?Sized + ToOwned> Cow<'_, B> {
     /// assert!(!bull.is_owned());
     /// ```
     #[unstable(feature = "cow_is_borrowed", issue = "65143")]
-    pub fn is_owned(&self) -> bool {
+    pub const fn is_owned(&self) -> bool {
         !self.is_borrowed()
     }