[Parser] Rename generated `.c` to `.cpp` files (#8894)

[Parser] Rename generated `.c` to `.cpp` files (#8894)

Using Clang on my machine, I am seeing the following warning:

```
clang-13: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
```

This is because we are treating the generated `.c` files as C++
files. To make our build system more robust, this commit
changes our command for generating files to rename the
generated files to use the `.cpp` extension.