Report "declaration should be marked with impl" when possible

Also support a quick fix to add 'impl' modifier (KT-18454), although it
doesn't work yet on classes because there's no error on them in the IDE

 #KT-18087 Fixed
 #KT-18452 Fixed
 #KT-18454

compiler/testData/multiplatform/simpleNoImplKeywordOnTopLevelFunction/common.kt

@@ -0,0 +1,3 @@
+package test
+
+header fun foo(s: String): Array<CharSequence?>

compiler/testData/multiplatform/simpleNoImplKeywordOnTopLevelFunction/jvm.kt

@@ -0,0 +1,3 @@
+package test
+
+fun foo(s: String): Array<CharSequence?> = arrayOf(s)

compiler/testData/multiplatform/simpleNoImplKeywordOnTopLevelFunction/output.txt

@@ -0,0 +1,10 @@
+-- Common --
+Exit code: OK
+Output:
+
+-- JVM --
+Exit code: COMPILATION_ERROR
+Output:
+compiler/testData/multiplatform/simpleNoImplKeywordOnTopLevelFunction/jvm.kt:3:1: error: declaration should be marked with 'impl' (suppress with -Xno-check-impl)
+fun foo(s: String): Array<CharSequence?> = arrayOf(s)
+^