Un-XFAIL new test on Linux. This doesn't crash on Linux.

Author: DougGregor

test/Index/Store/cross-file-extension-crash.swift

@@ -1,8 +1,6 @@
 // Ensure that we don't crash due to resolving an extension in a non-primary
 // file.
 
-// XFAIL: linux
-
 // RUN: %empty-directory(%t)
 // RUN: %target-swift-frontend -index-store-path %t/idx -o %t/file.o -typecheck -primary-file %s %S/Inputs/cross-file-extension-crash-other.swift -verify