[flang] Rename features.h because many linux systems have an include file...

[flang] Rename features.h because many linux systems have an include file called features.h which can complicate makefiles that use a search path.

Original-commit: flang-compiler/f18@fc34d0ddf355e4f978e2cb984486191041c24ea9
Reviewed-on: https://github.com/flang-compiler/f18/pull/826
Tree-same-pre-rewrite: false