Add support for macros with new swift_compiler_plugin rule

[keith]

This adds support for macros with a new swift_compiler_plugin rule. Example usage:

swift_compiler_plugin(
    name = "Macros",
    srcs = glob(["Macros/*.swift"]),
    deps = [
        "@SwiftSyntax",
        "@SwiftSyntax//:SwiftCompilerPlugin",
        "@SwiftSyntax//:SwiftSyntaxMacros",
    ],
)

swift_test(
    name = "MacrosTests",
    srcs = glob(["MacrosTests/*.swift"]),
    deps = [
        ":Macros",
        "@SwiftSyntax//:SwiftSyntaxMacrosTestSupport",
    ],
)

swift_library(
    name = "MacroLibrary",
    srcs = glob(["MacroLibrary/*.swift"]),
    plugins = [":Macros"],
)

swift_library(
    name = "MacroConsumer",
    srcs = glob(["Sources/*.swift"]),
    deps = [":MacroLibrary"],
)

[keith]

might need 69349ee

[brentleyjones]

I'll update my local usage of this tomorrow to test it out.

[brentleyjones]

When using this accidentally with an older swift version, the errors are super cryptic:

<unknown>:0: error: if any output files are specified, they all must be

I wonder if we can throw an error when using the plugins attribute with an invalid Xcode or swiftc version. Maybe we do something based on SWIFT_FEATURE__SUPPORTS_MACROS?

[brentleyjones]

Local testing worked.

[keith]

added a feature check inside swift_compiler_plugin to fail early in that case. that should be enough since it's the only thing that creates the provider required for use in plugins = [ ... ]