As part of https://github.com/llvm/llvm-project/pull/143177, I moved the
non-libc++ specific formatting of `std::string`s out to `CxxStringTypes`
as MSVC's STL `std::string` can also be thought of a pointer+size pair.
I named this kind of string "string buffer".
This PR picks that change, so the MSVC PR can be smaller.
Unfortunately, libstdc++'s `std::string` does not fit this (it also uses
a different string printer function).
This resolves two FIXMEs in the libc++ tests, where empty u16 and u32
strings didn't have any prefix (u/U).
b14e03d855