[Sema] Improve test coverage of narrowing conversion diagnostics

Summary:
This patch adds tests of narrowing conversion diagnostics for the
'unscoped enum -> integer' case.

Reviewers: faisalv, rsmith, rogfer01

Reviewed By: rogfer01

Subscribers: cfe-commits, rogfer01

Differential Revision: https://reviews.llvm.org/D43572

llvm-svn: 326551