RCORE-2140 Work around a VC++ bug in util::overload

[tgoyne]

util::overload hits https://developercommunity.visualstudio.com/t/runtime-stack-corruption-using-stdvisit/346200, which has been fixed in the latest version of Visual Studio but not the version we have installed on evergreen (including on the vsCurrent builders). We switched away from using it in one place in #4825 due to this bug, but the core tests also hit this when run with debug runtime checks enabled. This change works around the bug so that util::overload works.

[ironage]

Great work tracking down the source of this!

[coveralls-official]

Pull Request Test Coverage Report for Build thomas.goyne_368

Details

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• 42 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+0.04%) to 90.862%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/util/file.cpp	1	78.73%
test/test_index_string.cpp	1	93.48%
src/realm/sync/noinst/client_impl_base.cpp	2	82.48%
src/realm/table.cpp	3	90.14%
src/realm/unicode.cpp	3	83.83%
test/object-store/util/sync/baas_admin_api.cpp	5	84.93%
src/realm/bplustree.cpp	7	71.41%
src/realm/sync/noinst/server/server.cpp	9	73.93%
test/fuzz_group.cpp	11	47.05%

Totals
Change from base Build 2348:	0.04%
Covered Lines:	214718
Relevant Lines:	236312

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💛 - Coveralls