Implemented simplest version of unused symbol inspection.

idea/testData/inspections/unusedSymbol/class/inspectionData/expected.xml

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+<problems>
+  <problem>
+    <file>unused.kt</file>
+    <line>1</line>
+    <module>light_idea_test_case</module>
+    <entry_point TYPE="file" FQNAME="temp:///src/unused.kt" />
+    <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
+    <description>Class 'A' is never used</description>
+  </problem>
+  <problem>
+    <file>usedOnlyInside.kt</file>
+    <line>1</line>
+    <module>light_idea_test_case</module>
+    <entry_point TYPE="file" FQNAME="temp:///src/usedOnlyInside.kt" />
+    <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
+    <description>Class 'A' is never used</description>
+  </problem>
+  <problem>
+    <file>localUnused.kt</file>
+    <line>2</line>
+    <module>light_idea_test_case</module>
+    <entry_point TYPE="file" FQNAME="temp:///src/localUnused.kt" />
+    <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
+    <description>Class 'Local' is never used</description>
+  </problem>
+</problems>

idea/testData/inspections/unusedSymbol/class/inspectionData/inspections.test

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+// INSPECTION_CLASS: org.jetbrains.kotlin.idea.inspections.UnusedSymbolInspection

idea/testData/inspections/unusedSymbol/class/localUnused.kt

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+fun f() {
+    class Local {
+    }
+}

idea/testData/inspections/unusedSymbol/class/localUsed.kt

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+fun f() {
+    class Local {
+
+    }
+
+    Local()
+}

idea/testData/inspections/unusedSymbol/class/unused.kt

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+class A

idea/testData/inspections/unusedSymbol/class/used.kt

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+class A
+
+fun f(): A {
+}

idea/testData/inspections/unusedSymbol/class/usedOnlyInside.kt

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+class A {
+    fun f(other: A) {
+    }
+}