[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/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) {}