From feedback, replace the name `_debugDescription`, which was confusing because of the
underscore, with `lldbDescription`. This new name also indicates that this property may
contain [LLDB Summary Strings](https://lldb.llvm.org/use/variable.html#summary-strings).
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
