clang: missed optimization for an empty string

[diamante0018]

When initializing a std::string with "" (empty) clang misses the opportunity to optimize a lot of code away.
In a real example we came across when examing a function from libc++ with @philnik777 we found the loss can be quite substantial

God bolt demo:
https://godbolt.org/z/f8av3EYbn

I opened this issue as advised by philnik777