[MPP] Fix a failing MPP diagnostic test

This test had the IGNORE_FIR2IR_EXCEPTIONS_IF_FIR_CONTAINS_ERRORS
directive enabled. This prevented some diagnostics from showing up
in this test, because there was this exception thrown in FIR2IR:

java.lang.IllegalStateException: IrSimpleFunctionPublicSymbolImpl for
/foo|foo(){}[0] is already bound:
FUN name:foo visibility:public modality:FINAL <> () returnType:kotlin.Unit

a0ba878b1d disabled the
`linkViaSignatures` flag in JS tests, which also fixed the exception.
Because of that, the full pipeline could be run, and the diagnostics
started to appear, which is the correct behavior.

Not that this only concerns _tests_, not production code, because
in the production pipeline `linkViaSignatures` had been disabled even
before that change.

compiler/testData/diagnostics/tests/multiplatform/topLevelFun/conflictingImplDeclarations.fir.kt

@@ -1,7 +1,7 @@
 // MODULE: m1-common
 // FILE: common.kt
 
-<!AMBIGUOUS_ACTUALS{JVM}, NO_ACTUAL_FOR_EXPECT{JVM}!>expect fun foo()<!>
+<!AMBIGUOUS_ACTUALS{JVM}, AMBIGUOUS_ACTUALS{JS}, NO_ACTUAL_FOR_EXPECT{JVM}, NO_ACTUAL_FOR_EXPECT{JS}!>expect fun foo()<!>
 
 // MODULE: m2-jvm()()(m1-common)
 // FILE: jvm.kt