* Update for the breaking changes to std.fs.path.resolve. This had a
   happy side effect of deleting some error handling code which is no
   longer needed.
 * Introduce cache_exempt_flags field to CSourceFile. This is used only
   for include directories when building libc++ and libc++abi which
   depend only on the zig lib path.
 * libc_include_dir_list is only added to the cache hash when it
   contains directories which have been obtained from system probing. It
   is exempt when the directories depend only on the zig lib path.