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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98459 91177308-0d34-0410-b5e6-96231b3b80d8