[flang] fir.emboxchar codegen with static length fir.char types

The verifier of fir.emboxchar accepts `fir.char<kind, constant>` types,
but its codegen failed because of a missing cast. It is convenient and
safe to not require making the cast in FIR, so add it in codegen.

Differential Revision: https://reviews.llvm.org/D139514