StripSymbols: Use DIGlobalVariable::getConstant() instead of getGlobal()

The only difference between the two is a `dyn_cast<>` to
`GlobalVariable`.  If optimizations have left anything behind when a
global gets replaced, then it doesn't seem like the debug info is dead.

I can't seem to find an optimization that would leave behind a
non-`GlobalVariable` without nulling the reference entirely, so I
haven't added a testcase (but I'll be deleting `getGlobal()` in a future
commit).

llvm-svn: 234792