[flang][driver] Add default intrinsic module path in f18 to make f18 behave...

[flang][driver] Add default intrinsic module path in f18 to make f18 behave like flang-new (with respect to the module paths), make it possible to share more tests between the drivers and make using f18 easier (the default path means that users are no longer required to specify it)

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D99336