Add a general substitution mechanism to support substituting symbolic
bindings with their values throughout symbolic constants and, more
specifically, types. This is done by decomposing the constant
instruction into its operands, substituting into the operands, and then
rebuilding the constant value by invoking the constant evaluator.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>
