Set up a client-side C library to talk with C APIs exposed from the compiler

We could use functions like dlopen and dlsym in this client-side library to talk with
compiler-side C APIs.

Author: nkcsgexi

Package.swift

@@ -36,10 +36,12 @@ let package = Package(
       targets: ["SwiftDriverExecution"]),
   ],
   targets: [
+    .target(name: "_CSwiftDriver"),
+
     /// The driver library.
     .target(
       name: "SwiftDriver",
-      dependencies: ["SwiftOptions", "SwiftToolsSupport-auto", "Yams"]),
+      dependencies: ["SwiftOptions", "SwiftToolsSupport-auto", "Yams", "_CSwiftDriver"]),
 
     /// The execution library.
     .target(

Sources/CMakeLists.txt

@@ -6,6 +6,7 @@
 # See http://swift.org/LICENSE.txt for license information
 # See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 
+add_subdirectory(_CSwiftDriver/src)
 add_subdirectory(SwiftOptions)
 add_subdirectory(SwiftDriver)
 add_subdirectory(SwiftDriverExecution)

Sources/SwiftDriver/Driver/Driver.swift

@@ -13,6 +13,7 @@ import TSCBasic
 import TSCUtility
 import Foundation
 import SwiftOptions
+import _CSwiftDriver
 
 /// The Swift driver.
 public struct Driver {

Sources/_CSwiftDriver/include/_CSwiftDriver.h

@@ -0,0 +1 @@
+void dummyfunction();

Sources/_CSwiftDriver/src/_CSwiftDriverImpl.c

@@ -0,0 +1,4 @@
+#include "_CSwiftDriver.h"
+#include <dlfcn.h>
+
+void dummyfunction() {};