Sema: clean up peer resolution of errors

 * Fix compile error for `zirErrorUnionType`.
 * Convert zirMergeErrorSets logic to call `Type.errorSetMerge`.
   It does not need to create a Decl as the TODO comment hinted.
 * Extract out a function called `resolveInferredErrorSetTy`.
 * Rework `resolvePeerTypes` with respect to error unions and
   error sets. This is a less complex implementation that passes all the
   same tests and uses many fewer lines of code by taking advantage of
   the function `coerceInMemoryAllowedErrorSets`.
   - Always merge error sets in the order that makes sense, even when
     that means `@typeInfo` incompatibility with stage1.
 * `Type.errorSetMerge` no longer overallocates.
 * Don't skip passing tests.

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