On windows (PECOFF), the `static let` properties produced by `DebugDescriptionMacro`
are not constants, and as a result the `@_section` macro cannot be applied. The error
message is:
> global variable must be a compile-time constant to use `@_section` attribute
Until this issue is addressed, DebugDescriptionMacro is disabled for windows targets.
The data emitted by `DebugDescriptionMacro` is constant. For that reason, it was placed
in `__DATA_CONST`. However, this causes a problem with the linker, which emits an error
if the `__DATA_CONST` segment is _not_ marked `SG_READ_ONLY`.
After discussion, it was pointed out that if the constant data has no fixups, then it
can and should be in the the `__TEXT` segment. The `__DATA_CONST` segment is for data
that is essentially constant but contains dyld fixups.
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
