[flang] Refactoring

Eliminate two of the three overloadings of `Scope::MakeDerivedType()`.
Keep the one that has `Category` first because that's the order the
constructor of `DeclTypeSpec` uses.

In `mod-file.cc`, eliminate some calls to `PutLower()`. Symbols and
names are already lower case so they don't need to be converted.

In `modfile03.f90`, move the expected `.mod` file comments to right
after the corresponding modules. That makes it easier to work with.

Original-commit: flang-compiler/f18@86874d9bf8398391d3c24b0f0c8ed6c1e2c88185
Reviewed-on: https://github.com/flang-compiler/f18/pull/740
Tree-same-pre-rewrite: false