Plumb the information about the owning module and supplemental
signature modules through to the Macro data structure, for both
built-in and plugin macros.
We're resolving the given module names into module declarations, but
otherwise performing no checking and not emitting any diagnostics.
This information is not yet used.
Type check user-defined macros plugins with user-provided type signatures.
Also, load plugin libraries with `RTLD_LOCAL` instead of `RTLD_GLOBAL` to prevent symbol collision between plugins. `llvm::sys::DynamicLibrary` only supports `RTLD_GLOBAL` so we use the plain `dlopen` instead. This does not work on Windows and needs to be fixed.
Friend PR: apple/swift-syntax#1042
Allow user-defined macros to be loaded from dynamic libraries and evaluated.
- Introduce a _CompilerPluginSupport module installed into the toolchain. Its `_CompilerPlugin` protocol acts as a stable interface between the compiler and user-defined macros.
- Introduce a `-load-plugin-library <path>` attribute which allows users to specify dynamic libraries to be loaded into the compiler.
A macro library must declare a public top-level computed property `public var allMacros: [Any.Type]` and be compiled to a dynamic library. The compiler will call the getter of this property to obtain and register all macros.
Known issues:
- We current do not have a way to strip out unnecessary symbols from the plugin dylib, i.e. produce a plugin library that does not contain SwiftSyntax symbols that will collide with the compiler itself.
- `MacroExpansionExpr`'s type is hard-coded as `(Int, String)`. It should instead be specified by the macro via protocol requirements such as `signature` and `genericSignature`. We need more protocol requirements in `_CompilerPlugin` to handle this.
- `dlopen` is not secure and is only for prototyping use here.
Friend PR: apple/swift-syntax#1022
