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.
* [Observation] Forward availability and defines to extensions
* Simplify availability slightly from review feedback
* Simplify availability for extensions to use `.with`
* 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
Refactor the build support for ASTGen and ObservationMacros. Both have
nearly-identical copies of a bunch of messy, workaround-laden CMake
code to build "pure" Swift host libraries using CMake's Swift support.
Instead, introduce `add_pure_swift_host_library` to centralize all of
the hacks to build a host library that's all Swift, using CMake's
support directly (rather than custom commands), and link against the
swift-syntax stack. Switch ASTGen directly over to this.
Add `add_swift_macro_library` on top of this, to make it easy to
create a macro library and install it into the appopriate plugin
directory. Switch ObservationMacros over to this.
