Auto merge of rust-lang#130697 - bjoernager:const-char-make-ascii, r=dtolnay

Mark `char::make_ascii_uppercase` and `char::make_ascii_lowercase` as const.

Relevant tracking issue: rust-lang#130698

The `make_ascii_uppercase` and `make_ascii_lowercase` methods in `char` should be marked "const."

With the stabilisation of [`const_mut_refs`](rust-lang#57349), this simply requires adding the `const` specifier to the function signatures.

Author: bors

library/core/src/char/methods.rs

@@ -1279,8 +1279,9 @@ impl char {
     ///
     /// [`to_ascii_uppercase()`]: #method.to_ascii_uppercase
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_unstable(feature = "const_char_make_ascii", issue = "130698")]
     #[inline]
-    pub fn make_ascii_uppercase(&mut self) {
+    pub const fn make_ascii_uppercase(&mut self) {
         *self = self.to_ascii_uppercase();
     }
 
@@ -1304,8 +1305,9 @@ impl char {
     ///
     /// [`to_ascii_lowercase()`]: #method.to_ascii_lowercase
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_unstable(feature = "const_char_make_ascii", issue = "130698")]
     #[inline]
-    pub fn make_ascii_lowercase(&mut self) {
+    pub const fn make_ascii_lowercase(&mut self) {
         *self = self.to_ascii_lowercase();
     }