Add <atomic> include and fix OpenMP shared directive #1098

Job	Run time
python-lint	5s
clang-format	9s
ubuntu-gcc-build	0s
macos-clang-build	0s
windows-clang-build	0s
ubuntu-clang-test	0s
windows-msvc-build	0s
ubuntu-clang-build	0s
ubuntu-clang-sanitizer-build	0s
windows-clang-test	0s
ubuntu-gcc-test	0s
windows-msvc-test	0s
ubuntu-clang-test-extended	0s
macos-clang-test	0s
ubuntu-clang-sanitizer-test	0s
ubuntu-clang-sanitizer-test-extended	0s
windows-msvc-test-extended	0s
ubuntu-gcc-test-extended	-1s
ubuntu-gcc-build-codecov	-1s
ubuntu-gcc-build-perf-stats	-1s
macos-clang-test-extended	-1s
windows-clang-test-extended	-1s
	9s

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