Disable test that fails spuriously

This test fails sometimes with something like:

SOURCE_DIR/test/ModuleInterface/ModuleCache/prebuilt-module-cache-forwarding.swift:47:8: error: malformed compiled module: BUILD_DIR/test-macosx-x86_64/ModuleInterface/ModuleCache/Output/prebuilt-module-cache-forwarding.swift.tmp/MCP/Lib-2XZQCTV44N470.swiftmodule

rdar://58578342

Author: aschwaighofer

test/ModuleInterface/ModuleCache/prebuilt-module-cache-forwarding.swift

@@ -50,3 +50,5 @@ struct X {}
 let _: X = Lib.testValue
 // FROM-INTERFACE: [[@LINE-1]]:16: error: cannot convert value of type 'FromInterface' to specified type 'X'
 // FROM-PREBUILT: [[@LINE-2]]:16: error: cannot convert value of type 'FromPrebuilt' to specified type 'X'
+
+// REQUIRES: rdar58578342