Split out -verify into two distinct option flags

Instead of dual-purposing a single flag, rename the driver option to
--verify-debug-info.

The frontend -verify option that enables diagnostic verification remains
unchanged except that it's now a pure CC1Option.

Both have been given proper help text.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199451 91177308-0d34-0410-b5e6-96231b3b80d8