LLVM dialect conversion and target: support indirect calls

Add support for converting MLIR `call_indirect` instructions to the LLVM IR
dialect.  In LLVM IR, the same instruction is used for direct and indirect
calls.  In the dialect, we have `llvm.call` and `llvm.call0` to work around the
absence of the void type in MLIR.  For direct calls, the callee is stored as
instruction attribute.  Use the same pair of instructions for indirect calls by
omitting the callee attribute.  In the MLIR to LLVM IR translator, check the
presence of attribute to decide whether to construct a direct or an indirect
call using different LLVM IR Builder functions.

Add support for converting constants of function type to the LLVM IR dialect
and for translating them to the LLVM IR proper.  The `llvm.constant` operation
works similarly to other types: its attribute has MLIR function type but the
value it produces has LLVM IR function type wrapped in the dialect type.  While
lowering, look up the pointer to the converted function in the corresponding
mapping.

PiperOrigin-RevId: 234132351