[passes tests, mostly*] Remove Nonterminal::NtExpr.

[*] two ui tests temporarily removed, and one unit test has a couple of
failing-to-parse assertions removed.

XXX:
- Some tests have sub-optimal error messages, I want to improve them
- Added a `src` field to `Delimiter::Invisible` which indicates where it
  came from, partly replicates the Nonterminal kind within
  `Interpolated`.
- parse_meta_item_inner required significant changes, it was assuming
  that parse_unsuffixed_lit didn't consume any tokens on failure

Author: nnethercote

compiler/rustc_ast/src/ast.rs

@@ -1629,7 +1629,7 @@ impl MacDelimiter {
             Delimiter::Parenthesis => Some(MacDelimiter::Parenthesis),
             Delimiter::Bracket => Some(MacDelimiter::Bracket),
             Delimiter::Brace => Some(MacDelimiter::Brace),
-            Delimiter::Invisible => None,
+            Delimiter::Invisible(_) => None,
         }
     }
 }

compiler/rustc_ast/src/ast_traits.rs

@@ -233,7 +233,7 @@ impl HasTokens for Nonterminal {
         match self {
             Nonterminal::NtItem(item) => item.tokens(),
             Nonterminal::NtStmt(stmt) => stmt.tokens(),
-            Nonterminal::NtExpr(expr) | Nonterminal::NtLiteral(expr) => expr.tokens(),
+            Nonterminal::NtLiteral(expr) => expr.tokens(),
             Nonterminal::NtPat(pat) => pat.tokens(),
             Nonterminal::NtTy(ty) => ty.tokens(),
             Nonterminal::NtMeta(attr_item) => attr_item.tokens(),
@@ -247,7 +247,7 @@ impl HasTokens for Nonterminal {
         match self {
             Nonterminal::NtItem(item) => item.tokens_mut(),
             Nonterminal::NtStmt(stmt) => stmt.tokens_mut(),
-            Nonterminal::NtExpr(expr) | Nonterminal::NtLiteral(expr) => expr.tokens_mut(),
+            Nonterminal::NtLiteral(expr) => expr.tokens_mut(),
             Nonterminal::NtPat(pat) => pat.tokens_mut(),
             Nonterminal::NtTy(ty) => ty.tokens_mut(),
             Nonterminal::NtMeta(attr_item) => attr_item.tokens_mut(),