[ORC] Attempt to work around compile failure on some bots.

See e.g. https://lab.llvm.org/buildbot/#/builders/193/builds/98.

I think this failure is related to a C++ standard defect, 1397 --"Class
completeness in non-static data member initializers" [1]. If so, moving
to C++98 initialization should work around the issue.

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1397