ICE in ArrayType::calldataEncodedSize(bool) [Array size does not fit unsigned]

## Description

This contract:
```solidity
contract C {
struct R { uint[10][10] y; }
struct S { uint a; uint b; R d; uint[20][20][2999999999999999999999999990] c; }
function f() public {
C.S memory y;
C.S[10] memory z; y.a < 2;
}
}
```

produces:

```
Internal compiler error during compilation:
/home/user/solidity/libsolidity/ast/Types.cpp(1662): Throw in function virtual unsigned int solidity::frontend::ArrayType::calldataEncodedSize(bool) const
Dynamic exception type: boost::exception_detail::clone_impl
std::exception::what: Array size does not fit unsigned.
[solidity::util::tag_comment*] = Array size does not fit unsigned.
```

when compiled with solc

On master, using AFL fuzzing. Another discovery using https://github.com/agroce/afl-compiler-fuzzer, now with a few more mutation operators

## Environment

- Compiler version: 0.6.3-develop.2020.2.10+commit.e8eb1f2d.Linux.clang
- Target EVM version (as per compiler settings): N/A
- Framework/IDE (e.g. Truffle or Remix): N/A
- EVM execution environment / backend / blockchain client: N/A
- Operating system: Ubuntu 18.04 in docker

## Steps to Reproduce

Above shows pretty clearly, I think.

Reproduce the contract with solc, then inspect libsolidity/ast/Types.cpp around line 1662 and ArrayType::calldataEncodedSize(bool), where the reported ICE is thrown. Done means this input no longer produces the reported InternalCompilerError.

Written by the indexing model from the issue text.