[cxx-interop][SwiftCompilerSources] Include Clang headers

This will allow using Swift headers that include Clang headers from SwiftCompilerSources.

For example, some headers in `swift/Basic` include headers from `clang/Basic`. Currently adding those Swift headers to the modulemap causes a build error.

SwiftCompilerSources/Package.swift

@@ -22,6 +22,8 @@ private extension Target {
       "-Xcc", "-I", "-Xcc", "../include",
       // LLVM modules and headers
       "-Xcc", "-I", "-Xcc", "../../llvm-project/llvm/include",
+      // Clang modules and headers
+      "-Xcc", "-I", "-Xcc", "../../llvm-project/clang/include",
       "-cross-module-optimization"
     ]),
   ]