The original change was reverted because it was discovered
that clang mishandles thunks, and they receive wrong
attributes for their this/return types - the ones for the function
they will call, not the ones they have.
While i have tried to fix this in https://reviews.llvm.org/D100388
that patch has been up and stuck for a month now,
with little signs of progress.
So while it will be good to solve this for real,
for now we can simply avoid introducing the bug,
by not annotating this/return for thunks.
This reverts commit 6270b3a1ea,
relanding 0aa0458f14.
The OpenMP spec seems to require the compound operators be used for
+, *, &, |, and ^ reduction. So use these if a class has those operators.
If not try the simple operators as we did previously to limit the impact
to existing code.
Fixes: https://bugs.llvm.org/show_bug.cgi?id=48584
Differential Revision: https://reviews.llvm.org/D101941
Roman Lebedev