Change checkUnsafeAssignLiteral() to use the new Sema::CheckLiteralKind().

Along the way, fix a bug in CheckLiteralKind(), previously in diagnoseObjCLiteralComparison, where we didn't ignore parentheses
in boxed expressions for purpose of classification.

In other words, both @42 and @(42) should be classified as numeric
literals.

llvm-svn: 170931