[Bug libstdc++/79162] [7 Regression] [C++17] ambiguity in string assignment due to string_view overload

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79162

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Can you reduce the failing code down to something smaller than the entirety of
LLVM?

Richard also says the overload shouldn't exist and is a bug, but the overload
has to exist, because the C++17 draft is defective.