`<expected>`: Investigate reusing tail padding of the union

Given [[msvc::no_unique_address]] landed in Clang 18, we can applied it to union members (possibly with the whole union) in expected.

E.g. in this example

struct IC {
    int i;
    char c;
};
std::expected<IC, int> e;

its possible to make sizeof(e) == 8 because the size of the union is 8, while the first 5 bytes are value bytes, and bool flag can live in the tail padding of the union.

We need to ensure that the tail padding won't be furtherly reused - otherwise, when the expected object is itself potentially-overlapping, reconstruction of a union member may overwrite the data not controlled by the implementation.

See also LLVM-68552, LLVM-68733, and LLVM-69673.

No repository file or test is named; start by reading the expected implementation and the referenced LLVM-68552, LLVM-68733, and LLVM-69673 issues. Determine whether union tail padding can safely hold the flag and whether further reuse is prevented, then validate the proposed layout with the supplied IC example.