Long live the exception handling!

This patch fills the last necessary bits to enable exceptions
handling in LLVM. Currently only on x86-32/linux.

In fact, this patch adds necessary intrinsics (and their lowering) which
represent really weird target-specific gcc builtins used inside unwinder.

After corresponding llvm-gcc patch will land (easy) exceptions should be
more or less workable. However, exceptions handling support should not be 
thought as 'finished': I expect many small and not so small glitches
everywhere.

llvm-svn: 39855

llvm/lib/CodeGen/MachineModuleInfo.cpp

@@ -1476,6 +1476,8 @@ MachineModuleInfo::MachineModuleInfo()
 , FrameMoves()
 , LandingPads()
 , Personalities()
+, CallsEHReturn(0)
+, CallsUnwindInit(0)
 {
   // Always emit "no personality" info
   Personalities.push_back(NULL);
@@ -1523,6 +1525,8 @@ void MachineModuleInfo::EndFunction() {
   TypeInfos.clear();
   FilterIds.clear();
   FilterEnds.clear();
+  CallsEHReturn = 0;
+  CallsUnwindInit = 0;
 }
 
 /// getDescFor - Convert a Value to a debug information descriptor.