[Lexer] Allow '$x' to be parsed as identifier in debugger mode.
[Lexer] Allow '$x' to be parsed as identifier in debugger mode.

This is needed to support `print $0` in lldb where `$0` is the
shortcut for a closure argument. The associated lldb change
will be merged together with this one and will contain a test.

<rdar://problem/201719448>

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