Use different transformers per module in FIR diagnostics tests

Before this commit, we used same transformers for all modules,
which provoked using same ScopeSession for all modules.
Now we re-create transformers for any new module.
This fixes some problems with incorrect caching in ScopeSession.

NB: this provokes ambiguities in some old FE tests,
mostly they're correct (same as old FE).

compiler/testData/diagnostics/tests/multimodule/duplicateMethod/differentGenericsInParams.fir.kt

@@ -38,6 +38,6 @@ import p.*
 
 fun test(b: B?) {
     if (b is C) {
-        b?.foo(null, null)
+        b?.<!AMBIGUITY!>foo<!>(null, null)
     }
 }