`<flat_set>`: invariants on single element insertion

[AlexGuteniev]

I've fixed LLVM-tests, then we decided than tests were incorrect, and now the issue about incorrect tests is incorrect.

See:

• <flat_set>: more invariants fixes #5516
• Revert incorrect <flat_set> change, and update test failure reason #5522
• [libc++][test] flat_set std test uses EmplaceUnsafeContainer which is non-standard llvm/llvm-project#140448

I want this to be looked by someone who have not just paws, but the highest understanding.