[MS ABI] Rework .xdata HandlerType emission

Utilizing IMAGEREL relocations for synthetic IR constructs isn't
valuable, just clutter.  While we are here, simplify HandlerType names
by making the numeric value for the 'adjective' part of the mangled name
instead of appending '.const', etc.  The old scheme made for very long
global names and leads to wordy things like '.std_bad_alloc'

llvm-svn: 233503