[libc][NFC] Add a separate flag for capturing the '+' in fopen mode string.

Having a separate flag helps in setting up proper flags when
implementing, say the Linux specialization of File.

Along the way, a signature for a function which is to be used to open
files has been added. The implementation of the function is to be
included in platform specializations.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D121889