Use correct LanguageVersionSettings in MetadataSerializer

Also, require users of K2MetadataCompiler to pass "-Xmulti-platform"
manually. Gradle and Maven plugins already do that, so only users who
invoke kotlinc directly are going to be affected by this

 #KT-19287 Fixed

compiler/testData/multiplatform/compilerArguments/common.kt

@@ -0,0 +1,9 @@
+// ADDITIONAL_COMPILER_ARGUMENTS: -Xcoroutines=enable
+
+fun f(g: suspend () -> Unit): Any = g
+
+suspend fun h() = f { }
+
+header suspend fun k()
+
+header fun l(g: suspend () -> Unit): Any