Widen EHScope::ClenupBitFields::FixupDepth to avoid overflowing it (PR23490)

It currently only takes 2048 gotos to overflow the FixupDepth bitfield,
causing silent miscompilation. Apparently some parser generators run into
this (see PR).

I don't know that that data structure is terribly size sensitive anyway,
and since there's no room to widen the bitfield, let's just use a separate
word in EHCatchScope for it.

Differential Revision: http://reviews.llvm.org/D21566

llvm-svn: 273434