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/cli/js/jsExtraHelp.out

@@ -137,10 +137,11 @@ where advanced options include:
                              Eliminate builder inference restrictions, for example by allowing type variables to be returned from builder inference calls.
   -Xuse-fir-extended-checkers Use extended analysis mode based on the frontend IR.
                              Warning: This feature is not yet production-ready.
+  -Xuse-fir-fake-override-builder
+                             Generate all fake overrides via FIR2IR instead of IR, i.e. revert to behavior before KT-61514 was resolved.
   -Xuse-fir-ic               Compile using frontend IR internal incremental compilation.
                              Warning: This feature is not yet production-ready.
   -Xuse-fir-lt               Compile using the LightTree parser with the frontend IR.
-  -Xuse-ir-fake-override-builder Generate fake overrides via IR. See KT-61514
   -Xuse-k2                   Compile using the experimental K2 compiler pipeline. No compatibility guarantees are provided yet.
   -Xverbose-phases           Be verbose while performing the given backend phases.