[clang-tools-extra] [clang-tidy] Add a new check 'replace-with-string-view' (PR #172170)

[Baranov Victor via cfe-commits]

https://github.com/vbvictor commented:

Looks good, thanks for your work; however, I think we should add some options to tweak behavior before we can land it.

One major drawback is this pattern:

```diff
   template <typename T> std::string getArcana(const T &N) {
     return dump([&](TextNodeDumper &D) { D.Visit(N); });
   }
-  std::string getArcana(const NestedNameSpecifierLoc &NNS) { return ""; }
-  std::string getArcana(const TemplateName &NNS) { return ""; }
-  std::string getArcana(const CXXBaseSpecifier &CBS) { return ""; }
+  std::string_view getArcana(const NestedNameSpecifierLoc &NNS) { return ""; }
+  std::string_view getArcana(const TemplateName &NNS) { return ""; }
+  std::string_view getArcana(const CXXBaseSpecifier &CBS) { return ""; }
   std::string getArcana(const TemplateArgumentLoc &TAL) {
     return dump([&](TextNodeDumper &D) {
       D.Visit(TAL.getArgument(), TAL.getSourceRange());
     });
   }
   std::string getArcana(const TypeLoc &TL) {
     return dump([&](TextNodeDumper &D) { D.Visit(TL.getType()); });
   }
```

We have plenty of similar methods declared with `string` but some of them appear to use `std::string_view`. I'd want to leave `string` even in trivial cases for consistency.
So an option `AllowExplicitStringReturn` would allow manual `string` creation like this:
```cpp
std::string getArcana(const TemplateName &NNS) { 
return std::string{""}; // no warning
return std::string(""); // no warning
}
```
Instead of writing a bunch of `// NOLINT`
Note that `string("")` may conflict with `modernize-return-braced-init-list` but `string{}` won't.

Another drawback is:

```diff
-std::string toString(InvalidName::Kind K) {
+std::string_view toString(InvalidName::Kind K) {
   switch (K) {
   case InvalidName::Keywords:
     return "Keywords";
   llvm_unreachable("unhandled InvalidName kind");
 }
```

We probably want real `std::string` returned here, so I suggest the `IgnoredFunctions` option to handle this case (could be empty by default, and users would write their own functions like `to_string;toString`).
There are `IgnoredFunctions` in other options too.

https://github.com/llvm/llvm-project/pull/172170