Arbitrary order of contracts in override specifier is confusing.

Version: solidity ^0.6.3
Platform: http://remix.ethereum.org/#optimize=false&evmVersion=null&version=soljson-v0.6.3+commit.8dda9521.js

Issue: override(Two, One) doesn't seam to have the intended behavior. Instead, it retains the contract import inheritance.

Example:

```
pragma solidity ^0.6.3;

contract One {
uint256 public a;
event Hah(string message, uint a, uint b);
function foo() virtual public {
a = 1;
emit Hah("One::foo()", a, 0);
}
}

contract Two {
uint256 public b;
event Hah(string message, uint a, uint b);
function foo() virtual public {
b = 2;
emit Hah("Two::foo()", 0, b);
}
}

// Override contract One
contract OneAndTwo is One, Two {
event Hah(string message, uint a, uint b);
function foo() public override(One, Two) {
super.foo();
emit Hah("OneAndTwo::foo() after super.foo()", a, b);
}
}
// Calls Two.foo()

// Swap override contracts
contract OneAndTwoReversed is One, Two {
event Hah(string message, uint a, uint b);
function foo() public override(Two, One) {
super.foo();
emit Hah("OneAndTwoReversed::foo() after super.foo()", a, b);
}
}
// Still calls Two.foo()

// Swap import contracts
contract TwoAndOne is Two, One {
event Hah(string message, uint a, uint b);
function foo() public override(Two, One) {
super.foo();
emit Hah("TwoAndOne::foo() after super.foo()", a, b);
}
}
// Now calls One.foo()
```

Thanks to BokkyPooBah for workshopping this and providing the code examples

Start by reproducing the Solidity ^0.6.3 example in the linked Remix configuration, comparing the three inheritance and override-order cases. Review the compiler behavior for override specifiers and inheritance linearization; done means the intended ordering behavior is agreed and covered by an appropriate regression test or clarified in the issue.

Written by the indexing model from the issue text.