[FIR] varargs: do not leak source of value parameter to the call site

when source for compiled fir elements is set, it leaks e.g. to diagnostics,
which is then filtered

see `TestsWithStdLib.Experimental.testImplicitUsages`

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/BodyResolveUtils.kt

@@ -44,8 +44,9 @@ internal fun remapArgumentsWithVararg(
     var indexAfterVarargs = argumentList.size
     val newArgumentMapping = linkedMapOf<FirExpression, FirValueParameter>()
     val varargArgument = buildVarargArgumentsExpression {
-        this.varargElementType = varargParameterTypeRef.withReplacedConeType(varargElementType)
-        this.typeRef = varargParameterTypeRef.withReplacedConeType(varargArrayType)
+        //todo ideally we should use here a source from the use-site and not from the declaration-site
+        this.varargElementType = varargParameterTypeRef.withReplacedConeType(varargElementType, KtFakeSourceElementKind.VarargArgument)
+        this.typeRef = varargParameterTypeRef.withReplacedConeType(varargArrayType, KtFakeSourceElementKind.VarargArgument)
         for ((i, arg) in argumentList.withIndex()) {
             val valueParameter = argumentMapping.getValue(arg)
             // Collect arguments if `arg` is a vararg argument of interest or other vararg arguments.

compiler/testData/diagnostics/tests/unsignedTypes/conversions/conversionOfSignedToUnsigned.fir.kt

@@ -36,7 +36,7 @@ fun takeUInt(@ImplicitIntegerCoercion u: UInt) {}
 fun takeULong(@ImplicitIntegerCoercion u: ULong) {}
 
 @ExperimentalUnsignedTypes
-fun takeUBytes(@ImplicitIntegerCoercion vararg u: <!OPT_IN_USAGE!>UByte<!>) {}
+fun takeUBytes(@ImplicitIntegerCoercion vararg u: UByte) {}
 
 fun takeLong(@ImplicitIntegerCoercion l: Long) {}