Rename the 'concept' variable in SymbolInterfaces.td

`concept` is a reserved keyword in C++20, it can't be used as a variable name.
Here is an example of the failure:

```
      auto *concept = getInterfaceFor(op);
            ^
include/mlir/IR/SymbolInterfaces.h.inc:156:12: error: expected expression [clang-diagnostic-error]
      if (!concept)
           ^
```

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D99369