[cxx-interop][SwiftCompilerSources] Add LLVM headers to the include paths

This allows the Swift compiler to find headers like `llvm/Support/Compiler.h` that are used by Swift headers.

SwiftCompilerSources/Package.swift

@@ -20,6 +20,8 @@ private extension Target {
       "-Xfrontend", "-enable-cxx-interop",
       // Bridging modules and headers
       "-Xcc", "-I", "-Xcc", "../include",
+      // LLVM modules and headers
+      "-Xcc", "-I", "-Xcc", "../../llvm-project/llvm/include",
       "-cross-module-optimization"
     ]),
   ]