[FIR] Improve debugging of diagnostic ranges

`UNREACHABLE_CODE` is weird.
If we look into its
positioning strategy,
we'll see that this diagnostic
attempts to split the source
of its element into multiple ranges.
This is bad, because such smaller
ranges can easily overlap with other
diagnostics in such a way that
they are no longer hierarchical.
See the
`compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/3.kt` test
for an example of this behavior
(allow reporting `UNREACHABLE_CODE`
first). The problem will appear at
the condition of `case_7`.

compiler/test-infrastructure-utils/tests/org/jetbrains/kotlin/codeMetaInfo/CodeMetaInfoRenderer.kt

@@ -61,6 +61,12 @@ object CodeMetaInfoRenderer {
 
             while (current != null) {
                 val next: CodeMetaInfo? = if (iterator.hasNext()) iterator.next() else null
+                val outer = opened.lastOrNull()
+                if (outer != null) {
+                    require(current.end <= outer.end) {
+                        "The outer diagnostic ${outer.tag} at ${outer.start} ends at ${outer.end}, but the supposedly inner ${current?.tag} starting at ${current?.start} ends at ${current?.end}. Rendered so far:\n$builder"
+                    }
+                }
                 opened.push(current)
                 builder.append(current.asString())
                 when {