[Lexer] Disallow '$' as a start of identifier, special handle '$'

[rintaro]

I believe, accepting `$0` as an identifier was not intentional. It was not allowed in Swift2/3.0

let foo: (Int) -> Void = {
  let `$0` = "FOO"
  print($0, `$0`)
}

foo(42) // -> 42 FOO

This behavior was introduced in #5270 which intended to allow `$` as an identifier.

I think, we shouldn't generally consider $ as a valid start character for identifiers.
Instead, handle `$` specially.

[rintaro]

Ping, could someone please review this?
I believe the current behavior is actually harmful.

let `$0` = "FOO"
let foo: (Int) -> Void = {
  print($0, `$0`)
}

hits assertion.

virtual std::pair<bool, Expr *> (anonymous namespace)::Verifier::walkToExprPre(swift::Expr *):
Assertion `(HadError || !M.is<SourceFile*>() || M.get<SourceFile*>()->ASTStage < SourceFile::TypeChecked) && 
"OverloadedDeclRef" "in wrong phase"' failed.

[CodaFi]

Sorry about that, meant to approve this weeks ago. Thanks for fixing this.

@swift-ci please smoke test.

[rintaro]

No worries. Thanks @CodaFi !

Oops. It seems, somehow, I forgot to add testcases 👎 . I will.

[rintaro]

This is the Swift3 behavior. Diagnostics doesn't look nice.
We can improve this, but not for now.

[rintaro]

@swift-ci Please smoke test

[rintaro]

@swift-ci Please smoke test