[flang][driver][nfc] Fix capitalisation

As pointed out in https://reviews.llvm.org/D93401, some methods in the
Flang driver are named inconsistently. The driver strives to follow
Flang's C++ style [1] and this patch updates these methods accordingly.

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

Differential Revision: https://reviews.llvm.org/D118381