clean code: remove Deref<Target=RegionKind> impl for Region and use .kind()

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

compiler/rustc_symbol_mangling/src/v0.rs

@@ -368,7 +368,7 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
     }
 
     fn print_region(&mut self, region: ty::Region<'_>) -> Result<(), PrintError> {
-        let i = match *region {
+        let i = match region.kind() {
             // Erased lifetimes use the index 0, for a
             // shorter mangling of `L_`.
             ty::ReErased => 0,