The dependency of the Observation library on the observation macro
plugin introduces a direct dependency of a target library on a host
library, (lib -> stdlib) that we'd like to avoid. Instead, make the
Swift frontend binary depend on the macro plugin libraries that we
build, so that it's the complete host-side stack.
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.
