[flang][fir] Add the pre-code gen rewrite pass and codegen ops.
Before the conversion to LLVM-IR dialect and ultimately LLVM IR, FIR is
partially rewritten into a codegen form. This patch adds that pass, the
fircg dialect, and the small set of Ops in the fircg (sub) dialect.
Fircg is not part of the FIR dialect and should never be used outside of
the (closed) conversion to LLVM IR.
Authors: Eric Schweitz, Jean Perier, Rajan Walia, et.al.
Differential Revision: https://reviews.llvm.org/D98063