Enable -Wdangling-fields by default in Clang. I've run this warning over

a very large chunk of code and found zero false positives. I've only
found a few bugs, but that likely is because bugs of this nature
actually do manifest. We've also identified several bugs that were
caught by Valgrind, but would have been caught faster and more easily
with this warning.

If anyone has concerns, or this causes fallout on any build bots, lemme
know. I'm happy to just put it under -Wmost.

llvm-svn: 139112