JS DCE: drop unknown file report severity to WARNING

*.kjsm and other files might be received when FileCollection is
used in Gradle as a dependency.

Example: `testCompile project(":$coroutines_core").sourceSets.test.output`
(a popular-ish solution to introduce dependencies between tests)

compiler/testData/cli/js-dce/invalidFilename.args

@@ -0,0 +1,4 @@
+$TESTDATA_DIR$/invalidFilename.args
+$TESTDATA_DIR$/simple.js
+-output-dir
+$TEMP_DIR$/min

compiler/testData/cli/js-dce/invalidFilename.out

@@ -0,0 +1,2 @@
+warning: invalid file name '$TESTDATA_DIR$/invalidFilename.args'; must end either with '.js', '.zip' or '.jar'
+OK

compiler/testData/cli/js-dce/invalidFilename.test

@@ -0,0 +1 @@
+// EXISTS: min/simple.js