Using IncludeTree::FileList to concat the include tree file systems that are passed on the command-line. This significantly reduce the command-line size, and also makes the cache key computation a lot faster. rdar://148752988
#include "swift/Basic/Assertions.h"
ModuleAliasMap
-fixit-all
-emit-fixits-path
-emit-supported-features
-emit-supported-arguments
LockFileManager