[Build] Link XCTest with Foundation

In order to implement asynchronous tests, swift-corelibs-xctest
will take on a dependency upon swift-corelibs-foundation. To
properly link the two:

- Allow the package manager to take a path to a Foundation build.
- Extend the SWIFTPM_EXTRA_IMPORTS hack such that it can take a
  list of paths.

Author: modocache

Sources/Build/describe().swift

@@ -194,9 +194,20 @@ extension Product {
 }
 
 private func extraImports() -> [String] {
-    //FIXME HACK
-    if let I = getenv("SWIFTPM_EXTRA_IMPORT") {
-        return ["-I", I]
+    // FIXME: This is a hack to allow -I include paths to be passed through to
+    //        swift-build and swift-test invocations. SWIFTPM_EXTRA_IMPORTS is
+    //        a semicolon-delineated list of absolute paths, for example:
+    //
+    //        SWIFTPM_EXTRA_IMPORTS="/path/to/foo;/path/to/bar" swift-build
+    //
+    //        This should be fixed such that it does not rely upon environment
+    //        variables.
+    if let includes = getenv("SWIFTPM_EXTRA_IMPORTS") {
+        var args = [String]()
+        for include in includes.characters.split(";").map(String.init) {
+            args += ["-I", include]
+        }
+        return args
     } else {
         return []
     }

Utilities/bootstrap

@@ -544,13 +544,18 @@ def main():
     cmd.extend(["-Xlinker", "-rpath", "-Xlinker", embed_rpath])
 
     cmd = [os.path.join(sandbox_path, "bin", "swift-build")]
-    if args.xctest_path:
-        env_cmd.append("SWIFTPM_EXTRA_IMPORT=" + args.xctest_path)
+    if args.foundation_path and args.xctest_path:
+        core_foundation_path = os.path.join(
+            args.foundation_path, "usr", "lib", "swift")
+        imports = ";".join([
+            args.xctest_path, args.foundation_path, core_foundation_path])
+        env_cmd.append("SWIFTPM_EXTRA_IMPORTS=" + imports)
         # Tell the linker where to look for XCTest, but autolinking
         # knows to pass -lXCTest.
         cmd.extend(["-Xlinker", "-L", "-Xlinker", args.xctest_path])
         # Add an RPATH, so that the tests can be run directly.
         cmd.extend(["-Xlinker", "-rpath", "-Xlinker", args.xctest_path])
+        cmd.extend(["-Xlinker", "-rpath", "-Xlinker", args.foundation_path])
 
     cmd = env_cmd + cmd