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
Use FetchContent to include swift-syntax directly in swift. This can be
thought of as an `add_subdirectory` for a directory outside the root.
The default build directory will be `_deps/swiftsyntax-subbuild/`, though
the modules and shared libraries will be built in `lib/swift/host` by
passing down `SWIFT_HOST_LIBRARIES_DEST_DIR` to avoid copying them as we
were doing previously.
* Move `add_pure_swift_host_library()` from lib/CMakeLists.txt to
AddPureSwift.cmake so that code outside `lib` can use it
* Add `add_pure_swift_host_tool()` function to make a pure Swift
host executable target (for future usages)
* Specify depending `SwiftSyntax` modules explicitly because not all
Swift libraries uses all SwiftSyntax libraries
