[flang][nfc] Fix variable names in `FrontendOptions` & `PreprocessorOptions`

As all member variables in `FrontendOptions` and `PreprocessorOptions`
are public, we should be naming them as `variable` rather than
`variable_` [1]. This patch fixes that.

Also, `FrontendOptions` & `PreprocessorOptions` are re-defined as a
structs rather than classes (all fields are meant to be public).

[1]
https://github.com/llvm/llvm-project/blob/main/flang/docs/C%2B%2Bstyle.md#naming

Differential Revision: https://reviews.llvm.org/D107062