Add documentation/DriverAndFrontend.md

[peterwaller-arm]

Add documentation describing the driver and frontend. I've also linked to it from the top of the overview, which seemed like a logical place to link to it from.

This is a work in progress and a request for comments.

I'm happy for this documentation to be merged as is, and we intend to update it on a regular basis.

I'm away on leave until 14th Oct. My colleagues may reply in the meantime.

See also: #762 ("Add flang -fc1").

[CarolineConcatto]

Question:" On a conceptual level, it seems to me that having the new Flang frontend use existing Flang diagnostics code would be a more logical first step, so all of flang uses the same style of diagnostics." Reply: I believe we do not have ATM a class Flang Diagnostic implemented in Flang as it is in Clang. Most errors and messages that the compiler emits are with the method Message &Say. As discussed in the future Flang would need a Diagnostic Client as the one we have in Clang.

Question:"Obviously #762 is WIP, so is this a step you plan to take or would it be very hard to do this in the ex-clang frontend code and it would be easier to unify the two diagnostics handling code first?" Reply:Some progress of removing the dependency of Flang Driver Frontend from Clang Frontend was made in the https://github.com/flang-compiler/f18/pull/759. We intend to do similar for the https://github.com/flang-compiler/f18/pull/762 in the future.