[CaptureTracking] Fixup const correctness of DomTree arg (NFC)

Summary:
The PointerMayBeCapturedBefore function's DomTree arg should be
const instead of non-const. There are no non-const uses of it
in the function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330769 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed