This is an internal compiler argument for FIR which doesn't seem
valuable enough to be exposed in Gradle, because it'd require to perform
some kind of deprecation if it's changed or removed. It'll still be
possible to use it via `freeCompilerArgs += "-Xbuiltins-from-sources"`.
Note that generated Gradle options are not changed because the generator
was not rerun after adding this compiler argument. (And the test that
checks that the generator was run is failing right now.)
Alexander Udalov
0083ef053b