[flang] Fix type mismatch in verification error

FIR models Fortran intrinsic types with deliberate KIND values. Like
Fortran, COMPLEX and REAL have related KINDs in FIR. Lowering now
converts REAL types to floating point (MLIR) up front. This patch moves
the code to convert from FIR RealType to MLIR FloatType out of codegen
and into the builder, allowing FIR ComplexTypes to have their element
type returned as an MLIR FloatType.

We should consider whether to replace fir::ComplexType with
mlir::ComplexType at some point. I believe these types are presently
used to convey distinctins in the target ABIs in the Tilikum bridge
however.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D127636



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>