Make enum01/enum02/enum03 tests clang-compatible

... by entirely replacing the use of CPP by a custom preprocessor;
clang -E -traditional has no stringification mechanism at all.

Reviewed By: thomie, austin

Differential Revision: https://phabricator.haskell.org/D957

GHC Trac Issues: #9399

(cherry picked from commit b98ca17e)