Fixes the way `DebugDescriptionMacro` produces a regex type name.
The problem was use of backslash escapes that weren't sufficiently escaped. They needed
to be double escaped. To avoid this trap, the regexes now use `[.]` to match a dot,
instead of the more conventional `\.` syntax.
Some data types cannot modify their `description` or `debugDescription` properties, as
modification to those properties could result in breakage to their users. To support
these conditions, `DebugDescriptionMacro` should support and prioritize an independent
property (one that that is not being reused).
This change adds support for `_debugDescription`. The macro will now prioritize the
description properties in this order:
1. `_debugDescription`
2. `debugDescription`
3. `debugDescription`
rdar://120498021
Implementation of the DebugDescription macro pitched on the forums:
https://forums.swift.org/t/pitch-debug-description-macro/67711. In this initial commit,
the macro is named `_DebugDescription` to indicate it's internal use at this time,
pending Swift Evolution.
rdar://115180949
