[Macros] Fix handling of extension macro conformances and witnesses

[DougGregor]

Fix two inter-related issues with extension macros that provide conformances to a protocol, the combined effect of which is that one cannot meaningfully provide extension macros that implement conformances to a protocol like Equatable or Hashable that also supports auto-synthesis.

The first issue involves name lookup of operators provided by macro expansions. The logic for performing qualified lookup in addition to unqualified lookup (for operators) did not account for extension macros in the same manner as it did for member macros, so we would not find a macro-produced operator (such as operator==) in witness matching.

The second issue is more fundamental, which is that the conformance lookup table would create NormalProtocolConformance instances for pre-macro-expansion conformance entries, even though these should always have been superseded by explicit conformances within the macro expansion buffers. The end result is that we could end up with two NormalProtocolConformance records for the same conformance. Some code was taught to ignore the pre-expansion placeholder conformances, other code was not. Instead, we now refuse to create a NormalProtocolConformance for the pre-expansion entries, and remove all of the special-case checks for this, so we always using the superseding explicit conformances produced by the macro expansions (or error if the macros don't produce them).

Fixes rdar://113994346 / #66348

[DougGregor]

@swift-ci please smoke test

[hborla]

Not creating a ProtocolConformance for the pre-macro-expansion conformance entry is much cleaner than what I had done. Thank you!