[test] Pass wasi-sysroot to the %target-clang

`test/IRGen/sensitive.swift` failed on the WebAssembly bot because it
tries to include stdio.h, but `--sysroot` was not passed to the
`%target-clang`.

Author: kateinoigakukun

test/lit.cfg

@@ -1939,8 +1939,8 @@ elif run_os == 'wasi':
         '%s -emit-pcm -target %s' %
         (config.swiftc, config.variant_triple))
     config.target_clang = (
-        "%s -target %s %s -fobjc-runtime=ios-5.0" %
-        (config.clang, config.variant_triple, clang_mcp_opt))
+        "%s -target %s %s -fobjc-runtime=ios-5.0 --sysroot %s" %
+        (config.clang, config.variant_triple, clang_mcp_opt, config.variant_sdk))
     config.target_ld = (
         "%s -L%r" %
         (config.wasm_ld, make_path(test_resource_dir, config.target_sdk_name)))