syntax: Make _ an identifier

src/librustc/hir/mod.rs

@@ -214,7 +214,7 @@ impl LifetimeName {
         use self::LifetimeName::*;
         match *self {
             Implicit => keywords::Invalid.name(),
-            Underscore => Symbol::intern("'_"),
+            Underscore => keywords::UnderscoreLifetime.name(),
             Static => keywords::StaticLifetime.name(),
             Name(name) => name,
         }

src/librustc/ich/impls_syntax.rs

@@ -287,7 +287,6 @@ fn hash_token<'a, 'gcx, W: StableHasherResult>(
         token::Token::Pound |
         token::Token::Dollar |
         token::Token::Question |
-        token::Token::Underscore |
         token::Token::Whitespace |
         token::Token::Comment |
         token::Token::Eof => {}