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).
Introduces the first use of `@_DebugDescription` in the standard library, applying it
to `ObjectIdentifier`. In order to use the DebugDescription macro in the stdlib, the
following changes are required:
1. Compilation must reference the just built macros (ie `libSwiftMacros.dylib`), not
those from the SDK. This is addressed by adding an explicit `-external-plugin-path`
flag that overrides the defaults generated by the compiler (which uses SDK paths, where
the macro may or may not exist, and may not be the current version).
2. As DebugDescription uses `@_section`, compilation enables the `SymbolLinkageMarkers`
feature.
Note that the use of DebugDescription is conditionally enabled for the following
reasons:
1. Use is disabled in freestanding builds, where the stdlib macros are not built.
2. Use is temporarily disabled in Linux builds due to a dynamic loader issue that needs
further investigation
The dynamic loader error causing issues with Linux CI is:
> swift-plugin-server: error while loading shared libraries: libswiftGlibc.so: cannot
open shared object file: No such file or directory
This PR depended on #71639, #71588, and #71685.
