Swift normally looks for the Clang resource dir in a subdir/symlink of its own
resource dir. We provide a symlink to the Swift build-time Clang as a default
there, but we also here patch a check to try locate it via NIX_CC.

--- a/Sources/SwiftDriver/Jobs/Toolchain+LinkerSupport.swift
+++ b/Sources/SwiftDriver/Jobs/Toolchain+LinkerSupport.swift
@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+import Foundation
 import SwiftOptions
 
 import protocol TSCBasic.FileSystem
@@ -26,6 +27,13 @@ extension Toolchain {
     for targetInfo: FrontendTargetInfo,
     parsedOptions: inout ParsedOptions
   ) throws -> VirtualPath {
+    let env = ProcessInfo.processInfo.environment
+    if let nixCC = env["NIX_CC"] {
+      return try VirtualPath(path: nixCC)
+        .appending(components: "resource-root", "lib",
+                   targetInfo.target.triple.platformName(conflatingDarwin: true)!)
+    }
+
     return VirtualPath.lookup(targetInfo.runtimeResourcePath.path)
       .appending(components: "clang", "lib",
                  targetInfo.target.triple.platformName(conflatingDarwin: true)!)