[clang][dataflow] Use `ignoreCFGOmittedNodes()` in `setValue()`. (#78245)

This is to be consistent with `getValue()`, which also uses
`ignoreCFGOmittedNodes()`.

Before this fix, it was not possible to retrieve a `Value` from a "CFG
omitted"
node that had previously been set using `setValue()`; see the
accompanying test,
which fails without the fix.

I discovered this issue while running internal integration tests on
https://github.com/llvm/llvm-project/pull/78127.