Revert "Try simplifying LexicalScopes ownership again."

Speculatively reverting due to a suspicious failure on a Windows
buildbot.

This reverts commit 10c37a012ea11596d44cd9059fe09c959caf30c8.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208131 91177308-0d34-0410-b5e6-96231b3b80d8