[FIR] Implement UNDERSCORE_IS_RESERVED, UNDERSCORE_USAGE_WITHOUT_BACKTICKS diagnostics (psi only)

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CommonDeclarationCheckers.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.fir.analysis.cfa.FirPropertyInitializationAnalyzer
 import org.jetbrains.kotlin.fir.analysis.cfa.FirReturnsImpliesAnalyzer
 import org.jetbrains.kotlin.fir.analysis.checkers.cfa.FirControlFlowChecker
 import org.jetbrains.kotlin.fir.analysis.checkers.declaration.*
+import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirReservedUnderscoreDeclarationChecker
 
 object CommonDeclarationCheckers : DeclarationCheckers() {
     override val basicDeclarationCheckers: Set<FirBasicDeclarationChecker>
@@ -20,6 +21,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
             FirConflictsChecker,
             FirConflictingProjectionChecker,
             FirTypeConstraintsChecker,
+            FirReservedUnderscoreDeclarationChecker
         )
 
     override val memberDeclarationCheckers: Set<FirMemberDeclarationChecker>

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CommonExpressionCheckers.kt

@@ -15,6 +15,7 @@ object CommonExpressionCheckers : ExpressionCheckers() {
 
     override val basicExpressionCheckers: Set<FirBasicExpressionChecker>
         get() = setOf(
+            FirReservedUnderscoreExpressionChecker
         )
 
     override val qualifiedAccessCheckers: Set<FirQualifiedAccessChecker>
@@ -31,7 +32,7 @@ object CommonExpressionCheckers : ExpressionCheckers() {
             FirTypeParameterInQualifiedAccessChecker,
             FirSealedClassConstructorCallChecker,
             FirUninitializedEnumChecker,
-            FirFunInterfaceConstructorReferenceChecker,
+            FirFunInterfaceConstructorReferenceChecker
         )
 
     override val functionCallCheckers: Set<FirFunctionCallChecker>