Rollup merge of #136355 - GuillaumeGomez:proc-macro_add_value_retrieval_methods, r=Amanieu

Add `*_value` methods to proc_macro lib

This is the implementation of https://github.com/rust-lang/libs-team/issues/459.

It allows to get the actual value (unescaped) of the different string literals.

Part of https://github.com/rust-lang/rust/issues/136652.

r? libs-api

compiler/rustc_lexer/src/lib.rs

@@ -26,11 +26,13 @@
 // tidy-alphabetical-end
 
 mod cursor;
-pub mod unescape;
 
 #[cfg(test)]
 mod tests;
 
+// FIXME: This is needed for rust-analyzer. Remove this dependency once rust-analyzer uses
+// `literal-escaper`.
+pub use literal_escaper as unescape;
 use unicode_properties::UnicodeEmoji;
 pub use unicode_xid::UNICODE_VERSION as UNICODE_XID_VERSION;