Fix #10940 - Add support for degenerate cases to gammaIncomplete and gammaIncompleteCompl

[tedgin]

This satisfies #10940.

Previously gammaIncomplete hand inconsistent handling of a = 0 and real.infinity. This extends the handling of these two degenerate cases to all values of x >= 0. gammaIncompleteCompl was extended as well.

As part of this change, the NaN handling for these two functions was brought inline with the builtin infix operators, i.e., when both a and x are NaNs, the one with the larger payload is returned.

[dlang-bot]

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Testing this PR locally

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dub run digger -- build "master + phobos#10943"