[ClangImporter] Look for platform modulemaps in the resource dir first
This applies https://github.com/swiftlang/swift/pull/74814 for all targets except Android.
This fixes a common source of build failures on Linux, where shim functions from CxxStdlibShim would not be found by the hosttools Swift compiler when building the CxxStdlib overlay.
This change is keeping Android intact to keep the existing Android SDK working.