K2. Minor. Rename FirConstExpression to FirLiteralExpression

FirConstExpression is usually confused with "constant" calculations,
while in fact, it just denotes a simple literal expression
and `1 + 1` isn't represented by a FirConstExpression.

^KT-64314 Fixed

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/CFGNode.kt

@@ -668,7 +668,7 @@ class JumpNode(owner: ControlFlowGraph, override val fir: FirJump<*>, level: Int
         return visitor.visitJumpNode(this, data)
     }
 }
-class ConstExpressionNode(owner: ControlFlowGraph, override val fir: FirConstExpression<*>, level: Int) : CFGNode<FirConstExpression<*>>(owner, level) {
+class ConstExpressionNode(owner: ControlFlowGraph, override val fir: FirLiteralExpression<*>, level: Int) : CFGNode<FirLiteralExpression<*>>(owner, level) {
     override fun <R, D> accept(visitor: ControlFlowGraphVisitor<R, D>, data: D): R {
         return visitor.visitConstExpressionNode(this, data)
     }

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/diagnostics/ConeDiagnostics.kt

@@ -13,7 +13,7 @@ import org.jetbrains.kotlin.fir.declarations.FirVariable
 import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnostic
 import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnosticWithNullability
 import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnosticWithSource
-import org.jetbrains.kotlin.fir.expressions.FirConstExpression
+import org.jetbrains.kotlin.fir.expressions.FirLiteralExpression
 import org.jetbrains.kotlin.fir.expressions.FirOperation
 import org.jetbrains.kotlin.fir.expressions.FirThisReceiverExpression
 import org.jetbrains.kotlin.fir.render
@@ -199,7 +199,7 @@ sealed class ConeContractDescriptionError : ConeDiagnostic {
     }
 
     class IllegalConst(
-        val element: FirConstExpression<*>,
+        val element: FirLiteralExpression<*>,
         val onlyNullAllowed: Boolean
     ) : ConeContractDescriptionError() {
         override val reason: String