[ExplicitModule] Fix codegen target when using direct cc1 mode
When using direct-cc1 scanning mode, `-clang-target` is passed to
swift-frontend as `-Xcc` option, thus it causes swift IRGen to use
clang-target, instead of `-target` option of swift invocation. Teach
clang importer to restore target triple from swift for codegen option.
rdar://130547690
