Rename RunCallBacksToRun to llvm::sys::RunSignalHandlers

And expose it in Signals.h, allowing clients to call it directly,
possibly LLVMErrorHandler which currently calls RunInterruptHandlers
but not RunSignalHandlers, thus for example not printing the stack
backtrace on Unixish OSes. On Windows it does happen because
RunInterruptHandlers ends up calling the callbacks as well via 
Cleanup(). This difference in behaviour and code structures in
*/Signals.inc should be patched in the future.

llvm-svn: 242936