Factor out the handler work from SignalHandler into a helper function,

Factor out the handler work from SignalHandler into a helper function,
and change llvm::sys::RunInterruptHandlers to call that function directly
instead of calling SignalHandler, because the rest of SignalHandler
invokes side effects which aren't appropriate, including raising the
signal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104896 91177308-0d34-0410-b5e6-96231b3b80d8