Extend const_convert for Cow on From<&str> and From<String>

These implementations are trivially const - since they only create the enum structure
without running any non-const code.

const_convert is tracked by rust-lang#88674

Author: nipunn1313

library/alloc/src/string.rs

@@ -2701,7 +2701,8 @@ impl<'a> From<Cow<'a, str>> for String {
 
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a> From<&'a str> for Cow<'a, str> {
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<'a> const From<&'a str> for Cow<'a, str> {
     /// Converts a string slice into a [`Borrowed`] variant.
     /// No heap allocation is performed, and the string
     /// is not copied.
@@ -2722,7 +2723,8 @@ impl<'a> From<&'a str> for Cow<'a, str> {
 
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a> From<String> for Cow<'a, str> {
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<'a> const From<String> for Cow<'a, str> {
     /// Converts a [`String`] into an [`Owned`] variant.
     /// No heap allocation is performed, and the string
     /// is not copied.