Raise correct error with invalid pattern matching conditionals

[st0012]

We need to generate this error in a way that matches Sorbet's parser:

  case quux
# ^^^^ error: unexpected token "case"
  in x if 1, 2
  end

Currently it's translated into this without error:

CaseMatch {
  expr = Send {
    receiver = NULL
    method = <U quux>
    args = [
    ]
  }
  inBodies = [
    InPattern {
      pattern = MatchVar {
        name = <U x>
      }
      guard = IfGuard {
        condition = Integer {
          val = "1"
        }
      }
      body = Begin {
        stmts = [
          String {
            val = <U Unimplemented node type PM_MISSING_NODE (#103).>
          }
          Integer {
            val = "2"
          }
        ]
      }
    }
   ]
  elseBody = NULL
 }