[FIR] Avoid AST loading in KtPrimaryConstructor.toFirConstructor

- `getConstructorKeyword` forces an AST load due to a child search. In
  most cases, we don't need to check the constructor keyword in primary
  constructors at all (e.g. `class ABC()` with an implicit primary
  constructor). The keyword check is only needed when there are primary
  constructor modifiers (e.g. `class ABC constructor()`).

Thanks to Dima Gridin for the suggested fix.

^KT-61635 fixed

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/PsiRawFirBuilder.kt

@@ -1083,9 +1083,9 @@ open class PsiRawFirBuilder(
                 isFromSealedClass = owner.hasModifier(SEALED_KEYWORD) && explicitVisibility !== Visibilities.Private
                 isFromEnumClass = owner.hasModifier(ENUM_KEYWORD)
             }
-            val hasConstructorKeyword = this@toFirConstructor?.getConstructorKeyword() != null
+
             val builder = when {
-                this?.modifierList != null && !hasConstructorKeyword -> createErrorConstructorBuilder(ConeMissingConstructorKeyword)
+                this?.modifierList != null && getConstructorKeyword() == null -> createErrorConstructorBuilder(ConeMissingConstructorKeyword)
                 isErrorConstructor -> createErrorConstructorBuilder(ConeNoConstructorError)
                 else -> FirPrimaryConstructorBuilder()
             }