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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@234792 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,dslab-epfl/asap,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,apple/swift-llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm