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>
When declaring an associated entity in an interface -- just associated
functions for now -- create an associated entity value and corresponding
type to represent a "slot in a witness table". Also track the list of
associated entities on the interface so that we will eventually be able
to check impls against them.
Associated entities are represented as the integer index of their slot
in a witness table.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
