As all member variables in FrontendOptions are public, we should be
naming them as variable rather than variable_ [1]. This patch will
fix that.
Also, FrontendOptions is re-defined as a struct rather than a
class (all fields are meant to be public).
[1]
https://github.com/llvm/llvm-project/blob/main/flang/docs/C%2B%2Bstyle.md#naming
It is confusing to have data member initialization mixed between the constructor and the data member declarations when all of initialization expressions are constants.