[FIR] Part 5. Introduce paired common checkers for expect classes

There are some cases when we want to run some platform checker not from
  platform session but from common session. All such cases appear when
  we check some `expect` class

```kotlin
// MODULE: common
expect interface A
expect class B : A

class C : A

// MODULE: platform()()(common)
actual interface A {
    fun foo()
}

actual class B : A {
    override fun foo() {}
}
```

In this example we want to report "abstract foo not implemented" on
  class `C`, but we don't want to report it on `expect class B` (as
  its supertype is always `expect A`, never `actual A`)

So to cover such cases some platform checkers were split into two parts:
- `Regular`, which is platform checkers and runs for everything except
  expect declaration
- `ForExpectClass`, which is common checkers and runs only for expect
  declarations

^KT-58881 Fixed
^KT-58881 Fixed
^KT-64187 Fixed

compiler/fir/checkers/module.md

@@ -143,3 +143,30 @@ So the author of each new checker should decide in which session this checker sh
   it most likely should be `Common`
 - if the checker is interested in the scope of some type, it should be carefully considered how the actualization of the scope may affect
   the checker
+
+#### Checkers for expect classes
+
+```kotlin
+// MODULE: common
+expect interface A
+expect class B : A
+
+class C : A
+
+// MODULE: platform()()(common)
+actual interface A {
+    fun foo()
+}
+
+actual class B : A {
+    override fun foo() {}
+}
+```
+
+In this example we want to report "abstract foo not implemented" on  class `C`, but we don't want to report it on `expect class B` (as its supertype is always `expect A`, never `actual A`)
+
+So to cover such cases, it's worth splitting the platform checker into two parts:
+- `Regular`, which is platform checkers and runs for everything except `expect` declaration
+- `ForExpectClass`, which is common checkers and runs only for `expect` declarations
+
+As an example, check the implementation of [FirImplementationMismatchChecker](compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirImplementationMismatchChecker.kt) checker