[llvm] [llvm][Support] fix convertToSnakeFromCamelCase (PR #68375)

[via llvm-commits]

================
@@ -98,16 +98,18 @@ std::string llvm::convertToSnakeFromCamelCase(StringRef input) {
 
   std::string snakeCase;
   snakeCase.reserve(input.size());
-  for (char c : input) {
-    if (!std::isupper(c)) {
-      snakeCase.push_back(c);
-      continue;
-    }
-
-    if (!snakeCase.empty() && snakeCase.back() != '_')
+  auto check = [&input](size_t j, std::function<bool(int)> check) {
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rkayaith wrote:

nit
```suggestion
  auto check = [&input](size_t j, function_ref<bool(int)> check) {
```

https://github.com/llvm/llvm-project/pull/68375