This change introduces a new inst kind `CppFunctionPointerType`, which
represents an imported C++ function pointer that can be invoked from
Carbon (support for forming such a pointer from a Carbon function is in
a follow-up PR). This is implemented by treating the operation of
invoking a function pointer in C++ as if it were a call to an `__invoke`
method on the function pointer type, and extending the existing
function-import logic to support importing this fictitious method.
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Co-authored-by: Nicholas Bishop <nbishop@nbishop.net>