change EH related stuff (other than EH_LABEL) to use MCSymbol

instead of label ID's.  This cleans up and regularizes a bunch 
of code and makes way for future progress.

Unfortunately, this pointed out to me that JITDwarfEmitter.cpp
is largely copy and paste from DwarfException/MachineModuleInfo
and other places.  This is very sad and disturbing. :(

One major change here is that TidyLandingPads moved from being
called in DwarfException::BeginFunction to being called in
DwarfException::EndFunction.  There should not be any 
functionality change from doing this, but I'm not an EH expert.

llvm-svn: 98459