Fix a bug in my previous patch by restoring the behavior that the fatal
error handler is only registered once.

To avoid the use of std::call_once (the obvious way to do this) I've
wrapped everything up into a managed static and done the work in
a constructor. Silly, but it should be effective.

Some out-of-tree libclang users reported this to me, and I've asked them
to put together a test case which exhibits this behavior, but I wanted
to fix things ASAP since the nature of the fix is straight forward.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211905 91177308-0d34-0410-b5e6-96231b3b80d8