[CMake][NFC] Clean up CheckAtomic.cmake

`CheckAtomic.cmake` was skipping the test of whether atomics work in MSVC
without an atomics library (they do), but not setting the value of
`HAVE_CXX_ATOMICS_WITHOUT_LIB`. That caused build issues when trying to land
D69869. I fixed that issue in f128f442, by adding an `elseif(MSVC)`, as
was being done below in the 64-bit atomics check. That minimal fix did work,
but it kept various inconsistencies between the original atomics check and
the 64-bit one. This patch now makes the checks follow the same structure,
cleaning them up.

Differential Revision: https://reviews.llvm.org/D74767