Andrew Kelley 1a500b9699 Sema: avoid error return traces when possible
stage2 was adding bogus error return trace frames when an error was not
being returned. This commit makes several improvements:

 * Make a runtime check if necessary to only emit a frame into the error
   return trace when an actual error is returned.
 * Use the `analyzeIsNonErrComptimeOnly` machinery to avoid runtime
   checks when it is compile-time-known that the value is an error, or a
   non-error.
 * Make std.builtin.returnError take a non-optional stack trace pointer.

closes #12174
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..
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