Build 'lib/swift/host' libraries and linking binaries as a single
"package" instead of buillding the linking binaries with
"prefer-interface" hack.
This enables CMO between them.
Windows uses the runtime component for the DLL, the library component
for the import library, and archive component for static archives. This
is in contrast to Darwin that uses library for the DLL and archive for
static archives.
This is required to enable packaging the ObservableMacros.
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.
"Support Macros in Linux" patches was inconsistent with checking
platforms. Some only checked 'LINUX' but some matches
'LINUX|ANDROID|OPENBSD|FREEBSD'. Although I don't have tested other
platoforms than Linux at all, there's no reason to limit it to Linux.
So use the consistent check to match 'LINUX|ANDROID|OPENBSD|FREEBSD'
For compiling codes required for macro support, we now need swiftc
compiler in the build machine.
Unlike Darwin OSes, where swiftCore runtime is guaranteed to be present
in /usr/lib, Linux doesn't have ABI stability and the stdlib of the
build machine is not at the specific location. So the built compiler
cannot relies on the shared object in the toolchain.
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.
