Improve diagnostics on header/impl classes when scopes don't match

Try to report most of the errors on the actual members of the impl
class. In many cases, there's a 1:1 mapping of header to impl class
members, so the error "some members are not implemented" on the class
declaration itself is redundant. Exceptions include functions/properties
from supertypes (there may be no other place to report a signature
mismatch error in this case), functions/properties not marked with
'impl' (the checker is only run for declarations explicitly marked with
'impl') and default constructors (the checker is not run for them)

 #KT-18447 Fixed

compiler/testData/diagnostics/tests/multiplatform/headerClass/noImplKeywordOnMember.kt

@@ -0,0 +1,15 @@
+// !LANGUAGE: +MultiPlatformProjects
+// MODULE: m1-common
+// FILE: common.kt
+
+header class Foo {
+    fun bar(): String
+}
+
+// MODULE: m2-jvm(m1-common)
+// FILE: jvm.kt
+
+// TODO: run HeaderImplDeclarationChecker on non-impl members of impl classes, and report something like "impl expected" on 'bar' instead
+impl class <!HEADER_CLASS_MEMBERS_ARE_NOT_IMPLEMENTED!>Foo<!> {
+    fun bar(): String = "bar"
+}

compiler/testData/diagnostics/tests/multiplatform/headerClass/noImplKeywordOnMember.txt

@@ -0,0 +1,22 @@
+// -- Module: <m1-common> --
+package
+
+public final header class Foo {
+    public constructor Foo()
+    public final header fun bar(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+
+// -- Module: <m2-jvm> --
+package
+
+public final impl class Foo {
+    public constructor Foo()
+    public final fun bar(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}