Assume that a function is able to return a subtype if either (1) it is itself

Assume that a function is able to return a subtype if either (1) it is itself
marked with SQLITE_RESULT_SUBTYPE, or (2) one of its arguments is a function
that is able to return a subtype.  This check-in backs out the code changes
from the previous two on this same branch, but keeps the test cases from
the previous two.

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