[flang] Adapt to new directory for idioms.cc, idioms.h.

Change idioms.h so that calls to die will work outside the parser
namespace.

Use unordered_map to cache IntConst values.

Original-commit: flang-compiler/f18@9d06c385d9f2d9598f8e882d3badc9544c3567b9
Reviewed-on: https://github.com/flang-compiler/f18/pull/3