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`
There is no `%swift-frontend-target` subsitution in test, which is
actually `%target-swift-frontend`. The wrong spelling is actually
interpreted by lit as `%swift`-frontend-target, and surprising didn't
break any tests as the last argument from subsitution is
`-define-availability` so it just leads to an very akward availability
definition.
Instead of emitting an warning to the diagnostic engine, return the
plugin loading error as the result of the request. So that the user
can decide to emit it as a warning or an error.
