[flang] Add hlfir.concat codegen

hlfir.concat codegen happens in the "bufferization" pass: the pass
that place hlfir.expr in memory and transform all operations producing
or accepting hlfir.expr arguments into operations operating on memory.
Since this pass gets rid of the hlfir.expr type, this is a full
translation pass.

So far hlfir.concat codegen re-uses the current concatenation lowering helper
and only supports concatenation of two strings at a time. This matches
what lowering to HLFIR without any optimization produces. More optimized
versions will be developed later.

Differential Revision: https://reviews.llvm.org/D138673

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