Add new kind of pointer type for null-terminated array_ptrs

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#215

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To handle null-terminated strings, we will need to have null-terminated array_ptrs. The current plan is to have a new kind of type, just as we have ptr and array_ptr. The new type might be called nullterm_ptr. We still need to write up the design of the type. It will likely be modelled on the support for null-terminated pointers described in the Deputy research project. Programmers will be able to optionally declare that a variable with null-terminated pointer type has a bounds expression.

This proposed work items are based on what we did to implement array_ptr:

1. Extend the IR with a way to represent null-terminated array_ptr types. This includes extending AST dumping and pretty-printing.
2. Add parsing of null-terminated array_ptr types.
3. Restrict implicit conversions involving null-terminated array_ptr types:

• An array_ptr type cannot be converted implicitly to a null-terminated array_ptr type. This is because there could be another copy of the pointer that allows the null value to be overwritten.
• For similar reasons, unchecked pointer types cannot be converted implicitly to null-terminated array_ptr.
• Null-terminated pointer types can be implicitly converted to array_ptr types.
• Null terminated array_ptr pointers cannot be implicitly converted to an unchecked pointer type.

4. Write tests of language constructs using null-terminated array_ptr

• Expressions.
• Function calls

5. Extend existing interoperation support and write test of that.
6. Add bounds checking for null-terminated array_ptr types. The bounds checks will be different than those allowed for array_ptr types.

[secure-sw-dev-bot]

Comment from @dtarditi:

This feature is complete. There are open items for the remaining work:

• Disallow casts to null-terminated pointers in checked scopes. checkedc-clang#391
• Check array literals for null terminators. checkedc-clang#397
• Improve testing of null-terminated types, checked scopes checkedc-clang#455
• Infer widened bounds for pointers to null-terminated arrays checkedc-clang#467