[Accepted] SE-0402: Generalize `conformance` macros as `extension` macros

[xwu (Xiaodi Wu)]

[Update] Restrictions on Arbitrary Names at Global Scope in SE-0389 and SE-0397

Hi folks, After the acceptance of SE-0389 and SE-0397, further implementation work showed that allowing macros to introduce arbitrary names at global scope is fundamentally prone to creating cycles during type resolution. When performing unqualified name lookup, the compiler must expand all macros that produce arbitrary names, but such a macro may also have arguments that must be type-checked prior to expansion, and doing so would require expanding the very same macro. This limitation is not a…