Fir2Ir: enable IR-based fake override builder

Invert the logic of IR/FIR2IR-based generators: change the CLI argument
to -Xuse-fir-fake-override-builder, test directive to
USE_FIR_BASED_FAKE_OVERRIDE_GENERATOR, etc.

The changes in test data caused by using IR fake override builder by
default are in the subsequent commit.

 #KT-61514

compiler/tests-for-compiler-generator/tests/org/jetbrains/kotlin/test/generators/GenerateJUnit5CompilerTests.kt

@@ -333,7 +333,7 @@ fun generateJUnit5CompilerTests(args: Array<String>, mainClassName: String?) {
                 model("codegen/box")
             }
 
-            testClass<AbstractFirLightTreeBlackBoxCodegenWithIrFakeOverrideGeneratorTest> {
+            testClass<AbstractFirLightTreeBlackBoxCodegenWithFir2IrFakeOverrideGeneratorTest> {
                 model("codegen/box", excludeDirs = excludedScriptDirs)
             }