[DiagnosticInfo] Allow std::string_view in DiagnosticBuilder operator<<. (#190374)
After a68ae7b0cc0922b79114aabe8cf1ec8dc68524d7, calling `<<` with a
`std::string_view` gives:
```
clang/include/clang/Basic/Diagnostic.h:1319:8: error: use of overloaded operator '<<' is ambiguous (with operand types 'const StreamingDiagnostic' and 'const std::string_view')
1319 | DB << V;
| ~~ ^ ~
```