[clang] [-Wunsafe-buffer-usage] Fix AST matcher of UUCAddAssignGadget (PR #79392)

[via cfe-commits]

jkorous-apple wrote:

Buildkite failed for unrelated reason - this change really doesn't introduce virus in filecheck on Windows.

```
# executed command: 'c:\ws\src\build\build-clang-windows\bin\filecheck.exe' -check-prefix=CHECK-CC1 'C:\ws\src\clang\test\CodeCompletion\using.cpp'
--
  | # .---command stderr------------
  | # \| Could not create process (c:\ws\src\build\build-clang-windows\bin\filecheck.exe) due to [WinError 225] Operation did not complete successfully because the file contains a virus or potentially unwanted software
```

https://github.com/llvm/llvm-project/pull/79392