generate unchecked((int)0x91234567) instead of int.MinValue + 0x11234567

[TymurGubayev]

Problem

#1064
For &H80040E19 we get int.MinValue + 0x00040E19. With this PR, we'd get unchecked((int)0x80040E19) instead, thus preserving the hex literal value.

Solution

Corresponding code in LiteralConversion.cs was adjusted.

Additionally, I had to change CsWouldBeSimplifiedIncorrectly to return true for CastExpressionSyntax with Parent that's a CheckedExpressionSyntax, because otherwise it would've caused CS0266: "Cannot implicitly convert type 'uint' to 'int'".
Note this rule is a bit too aggressive:

int i = unchecked(0x80040E19); //we prevent error CS0266 here
uint u = unchecked(0x80040E19); //but we also prevent this from happening

(and many more cases with different types)

I'm reasonable sure it doesn't matter though: this PR introduces the very first call to SyntaxFactory.CheckedExpression.

[TymurGubayev]

the unit test that failed is unrelated, with following difference:

[System.Runtime.CompilerServices.CompilerGenerated()]
[global::System.Runtime.CompilerServices.CompilerGenerated()]

I had it locally once, and I think it's because of the modal dialog Debug.Fail wants to show (CachedReflectedDelegates.cs, line 60).

[TymurGubayev]

watched the unit test error some more, it just happens sporadically from time to time.

[GrahamTheCoder]

Interesting on the debug fall, I've never caught the root cause of that transient issue