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[CUDA][HIP] Fix implicit HD function resolution

Authored by yaxunl on Apr 24 2020, 1:41 PM.

Description

[CUDA][HIP] Fix implicit HD function resolution

recommit e03394c6a6ff with fix

When implicit HD function calls a function in device compilation,
if one candidate is an implicit HD function, current resolution rule is:

D wins over HD and H
HD and H are equal

this caused regression when there is an otherwise worse D candidate

This patch changes that to

D, HD and H are all equal

The rationale is that we already know for host compilation there is already
a valid candidate in HD and H candidates that will not cause error. Allowing
HD and H gives us a fall back candidate that will not cause error. If D wins,
that means D has to be a better match otherwise, therefore D should also
be a valid candidate that will not cause error. In this way, we can guarantee
no regression.

Differential Revision: https://reviews.llvm.org/D80450