Suppress -Wunused-variable for variables declared in headers, which may in
fact be defined and used in another TU.

Reshuffle some test cases because we suppress -Wunused-variable after we've
emitted an error.

This fixes PR15558.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179138 91177308-0d34-0410-b5e6-96231b3b80d8