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

llvm-svn: 226536