[FIR] Optimize performance and fix UNDERSCORE_IS_RESERVED, simplify code

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -2003,11 +2003,11 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val actualType: KtType
     }
 
-    abstract class UnderscoreIsReserved : KtFirDiagnostic<KtElement>() {
+    abstract class UnderscoreIsReserved : KtFirDiagnostic<PsiElement>() {
         override val diagnosticClass get() = UnderscoreIsReserved::class
     }
 
-    abstract class UnderscoreUsageWithoutBackticks : KtFirDiagnostic<KtElement>() {
+    abstract class UnderscoreUsageWithoutBackticks : KtFirDiagnostic<PsiElement>() {
         override val diagnosticClass get() = UnderscoreUsageWithoutBackticks::class
     }
 

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -3228,14 +3228,14 @@ internal class DelegateSpecialFunctionReturnTypeMismatchImpl(
 internal class UnderscoreIsReservedImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,
-) : KtFirDiagnostic.UnderscoreIsReserved(), KtAbstractFirDiagnostic<KtElement> {
+) : KtFirDiagnostic.UnderscoreIsReserved(), KtAbstractFirDiagnostic<PsiElement> {
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
 internal class UnderscoreUsageWithoutBackticksImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,
-) : KtFirDiagnostic.UnderscoreUsageWithoutBackticks(), KtAbstractFirDiagnostic<KtElement> {
+) : KtFirDiagnostic.UnderscoreUsageWithoutBackticks(), KtAbstractFirDiagnostic<PsiElement> {
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }