Prevent assertion violation when processing examples such as:
```
@[pattern] def badPattern (x : Nat) : Nat := 0
def tst (y : Nat) : Nat :=
match y with
| (@badPattern _) := 1
| _ := 2
```
The `x` is not used in `badPattern`. Thus, the elaborator fails to
synthesize the metavariable corresponding to `_` at `@badPattern _`.
The fix detects this kind of instance, but I commented the code the
throws the error because we would prevent us from compiling `term.lean`.
The assertion violation was originally triggered by the pattern definition
```
@[pattern] def «explicitBinderContent» (requireType : optParam.{1} Bool Bool.false) :=
{SyntaxNodeKind . name := `Lean.Parser.Term.explicitBinderContent}
at
...
view := fun stx, let (stx, i) := match stx.asNode : _ -> Prod Syntax Nat with
| some {kind := @«explicitBinderContent» requireType, -- << HERE
args := [stx], ..} := ...
```
These definitions were generated by the node choice macro.
cc @kha
cd21793b53