[clang-tidy] fix modernize-loop-convert to retain needed array-like operator[]

`modernize-loop-convert` handles //array-like// objects like vectors fairly well, but strips slightly too much information from the iteration expression by converting:
```
  Vector<Vector<int>> X;
  for (int J = 0; J < X[5].size(); ++J)
    copyArg(X[5][J]);
```
to
```
  Vector<Vector<int>> X;
  for (int J : X) // should be for (int J : X[5])
    copyArg(J);
```
The `[5]` is a call to `operator[]` and gets stripped by `LoopConvertCheck::getContainerString`. This patch fixes that and adds several test cases.

Reviewed By: njames93

Differential Revision: https://reviews.llvm.org/D95771