IR: Delete GenericDwarfNode during teardown

Fix a leak in `LLVMContextImpl` teardown that the leak sanitizer tracked
down [1].  I've just switched to automatic dispatch here (since I'll
inevitably forget again with the next class).

[1]: http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/811/steps/check-llvm%20asan/logs/stdio

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226536 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed