[FIR] Optimize performance and fix UNDERSCORE_IS_RESERVED, simplify code

compiler/testData/diagnostics/tests/Underscore.fir.kt

@@ -2,9 +2,9 @@
 
 import kotlin.Deprecated as <!UNDERSCORE_IS_RESERVED!>___<!>
 
-@___("") data class Pair(val x: Int, val y: Int)
+@<!UNDERSCORE_USAGE_WITHOUT_BACKTICKS!>___<!>("") data class Pair(val x: Int, val y: Int)
 
-class <!UNDERSCORE_IS_RESERVED, UNDERSCORE_IS_RESERVED!>_<!><<!UNDERSCORE_IS_RESERVED!>________<!>>
+class <!UNDERSCORE_IS_RESERVED!>_<!><<!UNDERSCORE_IS_RESERVED!>________<!>>
 val <!UNDERSCORE_IS_RESERVED!>______<!> = <!UNDERSCORE_USAGE_WITHOUT_BACKTICKS!>_<!><Int>()
 
 fun <!UNDERSCORE_IS_RESERVED!>__<!>(<!UNDERSCORE_IS_RESERVED!>___<!>: Int, y: <!UNDERSCORE_USAGE_WITHOUT_BACKTICKS!>_<!><Int>?): Int {
@@ -25,7 +25,7 @@ fun <!UNDERSCORE_IS_RESERVED!>__<!>(<!UNDERSCORE_IS_RESERVED!>___<!>: Int, y: <!
 }
 
 
-class A1(val <!UNDERSCORE_IS_RESERVED, UNDERSCORE_IS_RESERVED!>_<!>: String)
+class A1(val <!UNDERSCORE_IS_RESERVED!>_<!>: String)
 class A2(<!UNDERSCORE_IS_RESERVED!>_<!>: String) {
     class B {
         typealias <!UNDERSCORE_IS_RESERVED!>_<!> = CharSequence
@@ -48,3 +48,13 @@ val something2 = doIt { _ -> 1 }
 var p: Int?
     get() = null
     set(_) {}
+
+object `____` {
+    object Nested {
+        fun method() {}
+    }
+}
+
+fun test() {
+    <!UNDERSCORE_USAGE_WITHOUT_BACKTICKS!>____<!>.Nested.method()
+}

compiler/testData/diagnostics/tests/Underscore.kt

@@ -48,3 +48,13 @@ val something2 = doIt { _ -> 1 }
 var p: Int?
     get() = null
     set(_) {}
+
+object `____` {
+    object Nested {
+        fun method() {}
+    }
+}
+
+fun test() {
+    <!UNDERSCORE_USAGE_WITHOUT_BACKTICKS!>____<!>.Nested.method()
+}

compiler/testData/diagnostics/tests/Underscore.txt

@@ -7,6 +7,7 @@ public val something2: kotlin.Any?
 public fun __(/*0*/ ___: kotlin.Int, /*1*/ y: _<kotlin.Int>?): kotlin.Int
 public fun doIt(/*0*/ f: (kotlin.Any?) -> kotlin.Any?): kotlin.Any?
 public fun oneUnderscore(/*0*/ <anonymous parameter 0>: kotlin.Int): kotlin.Unit
+public fun test(): kotlin.Unit
 
 public final class A1 {
     public constructor A1(/*0*/ _: kotlin.String)
@@ -53,3 +54,18 @@ public final class _</*0*/ ________> {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
+public object ____ {
+    private constructor ____()
+    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
+
+    public object Nested {
+        private constructor Nested()
+        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 final fun method(): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
+

compiler/testData/diagnostics/tests/UnderscoreUsageInAnnotation.fir.kt

@@ -1,6 +0,0 @@
-package test
-
-annotation class `__`(val value: String)
-
-@__("") class TestAnn
-@`__`("") class TestAnn2

compiler/testData/diagnostics/tests/UnderscoreUsageInAnnotation.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 package test
 
 annotation class `__`(val value: String)