Adjacent scalars should always be coalesced

[hamishknight]

This applies to the DSL, for example:

    let r3 = Regex {
      "👨" as UnicodeScalar
      "\u{200D}" as UnicodeScalar
      "👨" as UnicodeScalar
      "\u{200D}" as UnicodeScalar
      "👧" as UnicodeScalar
      "\u{200D}" as UnicodeScalar
      "👦" as UnicodeScalar
    }

should match "👨‍👨‍👧‍👦", but currently doesn't. It also applies to custom character classes in regex literals, e.g [e\u{301}] should match e\u{301}, but currently doesn't.

rdar://96942688