Make constant emission non-recursive, and stop building a bogus
FunctionContext to emit constants.
To support this, move `InstConstantKind` from the typed instruction
definition into the `.def` file, and add more macros to allow us to
generate case labels based on whether an instruction is a constant.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>
