[clang][ConstExprEmitter] handle IntegerLiterals

Improves the ability of ConstExprEmitter to evaluate constants.

Found by adding asserts to ConstantEmitter::tryEmitPrivate to find cases
where ConstExprEmitter::Visit() fails to resolve (obvious) constants.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D156154