[FIR] Rename FirElvisCall to FirElvisExpression

FirElvisCall is not a FirCall, so FirElvisExpression is more
  convenient name

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/BuilderConfigurator.kt

@@ -287,7 +287,7 @@ object BuilderConfigurator : AbstractBuilderConfigurator<FirTreeBuilder>(FirTree
             useTypes(stubReferenceType)
         }
 
-        builder(elvisCall) {
+        builder(elvisExpression) {
             default("calleeReference", "FirStubReference")
             useTypes(stubReferenceType)
         }

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/FirTreeBuilder.kt

@@ -91,7 +91,7 @@ object FirTreeBuilder : AbstractFirTreeBuilder() {
     val qualifiedAccessWithoutCallee = element("QualifiedAccessWithoutCallee", Expression, statement)
     val qualifiedAccess = element("QualifiedAccess", Expression, qualifiedAccessWithoutCallee, resolvable)
     val checkNotNullCall = element("CheckNotNullCall", Expression, expression, call, resolvable)
-    val elvisCall = element("ElvisCall", Expression, expression, resolvable)
+    val elvisExpression = element("ElvisExpression", Expression, expression, resolvable)
 
     val arrayOfCall = element("ArrayOfCall", Expression, expression, call)
     val arraySetCall = element("AugmentedArraySetCall", Expression, statement)

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -187,7 +187,7 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
             needTransformOtherChildren()
         }
 
-        elvisCall.configure {
+        elvisExpression.configure {
             +field("lhs", expression).withTransform()
             +field("rhs", expression).withTransform()
         }