[leaks] Fix a leak recently introduced to the pragma handling. This
whole code would be better with std::unique_ptr managing the lifetimes
of the handlers, but I wanted to make a targeted fix to the leaks first.
With this change, all of the Clang preprocessor tests are leak free with
LSan.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207872 91177308-0d34-0410-b5e6-96231b3b80d8