[FIR] Add inline checker for bodies of inline functions

This checker doesn't support `@PublishedAPI` yet, so some BB tests for it
 were muted. #KT-46270

compiler/testData/diagnostics/tests/inline/stringTemplate.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_EXPRESSION
 inline public fun reg(converter: (Any) -> Any, str: String) {
     str
@@ -7,4 +8,4 @@ inline public fun reg(converter: (Any) -> Any, str: String) {
 public inline fun register(converter: (Any) -> Any) {
     "123$<!USAGE_IS_NOT_INLINABLE!>converter<!>"
     reg(converter, "123$<!USAGE_IS_NOT_INLINABLE!>converter<!>")
-}
+}