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/testData/ir/irText/declarations/fakeOverrideModality.kt

@@ -1,5 +1,4 @@
 // FIR_IDENTICAL
-// ENABLE_IR_FAKE_OVERRIDE_GENERATION
 // TARGET_BACKEND: JVM
 
 open class Base