[PackageLoading] Allow using Foundation in PackageDescription

This will add the arguments that are required to import Foundation in
PackageDescription library when bootstrapping on the CI. It should be
now possible to use Codable for generating JSON in the runtime
libraries.

<rdar://problem/45975602> Allow importing Foundation in the PackageDescription library

Author: aciidgh

Sources/PackageLoading/ManifestLoader.swift

@@ -317,6 +317,7 @@ public final class ManifestLoader: ManifestLoaderProtocol {
       #endif
         cmd += [resources.swiftCompiler.asString]
         cmd += ["--driver-mode=swift"]
+        cmd += bootstrapArgs()
         cmd += verbosity.ccArgs
         cmd += ["-L", runtimePath, "-lPackageDescription"]
         cmd += interpreterFlags
@@ -347,6 +348,36 @@ public final class ManifestLoader: ManifestLoaderProtocol {
         return json
     }
 
+    /// Returns the extra manifest args required during SwiftPM's own bootstrap.
+    private func bootstrapArgs() -> [String] {
+      #if !os(Linux)
+        return []
+      #else
+        // The Linux bots require extra arguments in order to locate the corelibs.
+        // We can potentially drop this by installing some stable linux toolchain 
+        // after Swift gets ABI and module stability.
+        //
+        // Compute if SwiftPM is bootstrapping.
+        let env = ProcessInfo.processInfo.environment
+        guard env.keys.contains("SWIFTPM_BOOTSTRAP") else { return [] }
+        guard let buildPathStr = env["SWIFTPM_BUILD_DIR"], let buildPath = try? AbsolutePath(validating: buildPathStr) else { 
+            return [] 
+        }
+
+        // Construct the required search paths relative to the build directory.
+        let libdir = buildPath.appending(RelativePath(".bootstrap/lib/swift/linux"))
+        let incdir = libdir.appending(component: "x86_64")
+        let dispatchIncdir = incdir.appending(component: "dispatch")
+
+        return [
+            "-I" + incdir.asString,
+            "-I" + dispatchIncdir.asString,
+            "-L" + libdir.asString,
+            "-Xcc", "-F" + incdir.asString,
+        ]
+      #endif
+    }
+
     /// Returns path to the sdk, if possible.
     private func sdkRoot() -> AbsolutePath? {
         if let sdkRoot = _sdkRoot {