[CMake] Fix swift_supports_implicit_module failure

swift_supports_implicit_module invokes /path/to/toolchain/usr/bin/swiftc
directly without using `xcrun`. That fails to set necessary
environmental variables to make swiftc infer the SDK path. Because of
that swift_supports_implicit_module used to fail because the test
compilation fails to load the stdlib. This PR workaround it by passing
`-parse-stdlib` to avoid implicit stdlib loading.

Author: rintaro

cmake/modules/SwiftImplicitImport.cmake

@@ -5,6 +5,7 @@ function(swift_supports_implicit_module module_name out_var)
     COMMAND
       "${CMAKE_Swift_COMPILER}"
       -Xfrontend -disable-implicit-${module_name}-module-import
+      -Xfrontend -parse-stdlib
       -c - -o /dev/null
     INPUT_FILE
       "${CMAKE_BINARY_DIR}/tmp/empty-check-${module_name}.swift"