[clang-tools-extra] [clang-tidy] Add `llvm-formatv-string` (PR #195974)

[via cfe-commits]

github-actions[bot] wrote:

<!--LLVM CODE LINT COMMENT: clang-tidy-->


:warning: C/C++ code linter, clang-tidy found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
</summary>

```bash

git diff -U0 origin/main...HEAD -- clang-tools-extra/clang-tidy/llvm/FormatvStringCheck.cpp clang-tools-extra/clang-tidy/llvm/FormatvStringCheck.h clang-tools-extra/clang-tidy/llvm/LLVMTidyModule.cpp |
python3 clang-tools-extra/clang-tidy/tool/clang-tidy-diff.py   -path build -p1 -quiet
```

</details>

<details>
<summary>
View the output from clang-tidy here.
</summary>

```


clang-tools-extra/clang-tidy/llvm/FormatvStringCheck.cpp:56:5: warning: variable 'Content' of type 'llvm::StringRef' can be declared 'const' [misc-const-correctness]
   56 |     llvm::StringRef Content = Fmt.substr(1, CloseBrace - 1);
      |     ^
      |                     const 
clang-tools-extra/clang-tidy/llvm/FormatvStringCheck.cpp:162:3: warning: variable 'FmtStr' of type 'llvm::StringRef' can be declared 'const' [misc-const-correctness]
  162 |   llvm::StringRef FmtStr = FmtLiteral->getString();
      |   ^
      |                   const 
clang-tools-extra/clang-tidy/llvm/FormatvStringCheck.cpp:187:10: warning: variable 'Index' of type 'unsigned int' can be declared 'const' [misc-const-correctness]
  187 |     for (unsigned Index : Parsed.Indices)
      |          ^
      |                   const
```

</details>


https://github.com/llvm/llvm-project/pull/195974