Report warning on @JvmStatic in private companion objects

Deprecation cycle to prohibit in 1.5.
^KT-25114 In Progress

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -285,6 +285,7 @@ public interface Errors {
     DiagnosticFactory1<PsiElement, String> EXPERIMENTAL_UNSIGNED_LITERALS_ERROR = DiagnosticFactory1.create(ERROR);
 
     DiagnosticFactory0<PsiElement> NON_PARENTHESIZED_ANNOTATIONS_ON_FUNCTIONAL_TYPES = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<PsiElement> JVM_STATIC_IN_PRIVATE_COMPANION = DiagnosticFactory0.create(WARNING);
 
     // Const
     DiagnosticFactory0<PsiElement> CONST_VAL_NOT_TOP_LEVEL_OR_OBJECT = DiagnosticFactory0.create(ERROR);

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -169,6 +169,7 @@ public class DefaultErrorMessages {
         MAP.put(EXPERIMENTAL_UNSIGNED_LITERALS_ERROR, "{0}", STRING);
 
         MAP.put(NON_PARENTHESIZED_ANNOTATIONS_ON_FUNCTIONAL_TYPES, "Non-parenthesized annotations on function types without receiver aren't yet supported (see KT-31734 for details)");
+        MAP.put(JVM_STATIC_IN_PRIVATE_COMPANION, "@JvmStatic is prohibited in private companion objects. This warning will become an error in the next major version");
 
         MAP.put(REDUNDANT_MODIFIER, "Modifier ''{0}'' is redundant because ''{1}'' is present", TO_STRING, TO_STRING);
         MAP.put(REDUNDANT_OPEN_IN_INTERFACE, "Modifier 'open' is redundant for abstract interface members");