SwiftCompilerSources: use unsafeFlags for header searches

As the headers are not part of the Swift package, the safe header search
option cannot be used.  The use of the unsafe flags enables building the
package on Windows.

SwiftCompilerSources/Package.swift

@@ -26,14 +26,14 @@ private extension Target {
         path: path ?? "Sources/\(name)",
         exclude: ["CMakeLists.txt"],
         sources: sources,
-        cxxSettings: [
-          .headerSearchPath("../include"),
-          .headerSearchPath("../../llvm-project/llvm/include"),
-          .headerSearchPath("../../llvm-project/clang/include"),
-        ],
         swiftSettings: [
           .interoperabilityMode(.Cxx),
-          .unsafeFlags(["-cross-module-optimization"]),
+          .unsafeFlags([
+            "-Xcc", "-I../include",
+            "-Xcc", "-I../../llvm-project/llvm/include",
+            "-Xcc", "-I../../llvm-project/clang/include",
+            "-cross-module-optimization",
+          ]),
         ] + swiftSettings)
     }
 }