Now that API descriptions are emitted during module build jobs when
`-emit-api-descriptor-path` is specified and the build system has been updated
to pass that flag when the output is needed, the `swift-api-extract` frontend
alias is no longer used. Delete it and the tests that were specific to invoking
`swift-api-extract`.
Resolves rdar://116537394.
Separate swift-syntax libs for the compiler and for the library plugins.
Compiler communicates with library plugins using serialized messages
just like executable plugins.
* `lib/swift/host/compiler/lib_Compiler*.dylib`(`lib/CompilerSwiftSyntax`):
swift-syntax libraries for compiler. Library evolution is disabled.
* Compiler (`ASTGen` and `swiftIDEUtilsBridging`) only depends on
`lib/swift/host/compiler` libraries.
* `SwiftInProcPluginServer`: In-process plugin server shared library.
This has one `swift_inproc_plugins_handle_message` entry point that
receives a message and return the response.
* In the compiler
* Add `-in-process-plugin-server-path` front-end option, which specifies
the `SwiftInProcPluginServer` shared library path.
* Remove `LoadedLibraryPlugin`, because all library plugins are managed
by `SwiftInProcPluginServer`
* Introduce abstract `CompilerPlugin` class that has 2 subclasses:
* `LoadedExecutablePlugin` existing class that represents an
executable plugin
* `InProcessPlugins` wraps `dlopen`ed `SwiftInProcPluginServer`
* Unified the code path in `TypeCheckMacros.cpp` and `ASTGen`, the
difference between executable plugins and library plugins are now
abstracted by `CompilerPlugin`
