LowerTypeTests: Change a few vtable globals in tests to constants.

LowerTypeTests: Change a few vtable globals in tests to constants.

It turns out that some of our negative tests were not in fact providing the
test coverage we expected: they were passing because the vtables were failing
an early check that they were constant. Fix this by changing the globals in
these tests to constants.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294550 91177308-0d34-0410-b5e6-96231b3b80d8