Return platform independent path in IncrementalCacheImpl::getClassFilePath

Fix ExperimentalIncrementalJpsTestGenerated.PureKotlin.testFunctionBecameInline on Windows.

jps-plugin/testData/incremental/pureKotlin/inlinFunctionUsageAdded/build.log

@@ -0,0 +1,7 @@
+Cleaning output files:
+out/production/module/META-INF/module.kotlin_module
+out/production/module/UsageKt.class
+End of files
+Compiling files:
+src/usage.kt
+End of files

jps-plugin/testData/incremental/pureKotlin/inlinFunctionUsageAdded/foo.kt

@@ -0,0 +1,5 @@
+class A {
+    inline fun foo(f: () -> Unit) {
+        f()
+    }
+}

jps-plugin/testData/incremental/pureKotlin/inlinFunctionUsageAdded/usage.kt

@@ -0,0 +1,2 @@
+fun usage() {
+}

jps-plugin/testData/incremental/pureKotlin/inlinFunctionUsageAdded/usage.kt.new

@@ -0,0 +1,3 @@
+fun usage() {
+    A().foo {}
+}