[flang][hlfir] Turn fir.char<1> results into hlfir.expr<fir.char<1>>

This gets rid of a special case with CHAR() intrinsic and BIND(C) results.
I tested this has no impact on the LLVM assembly when LLVM opt -01 or
more is run.
See comment in the patch for more details.

Differential Revision: https://reviews.llvm.org/D143270