Test .seh_startchained and .seh_endchained parsing.

Rework how the MCWin64EHUnwindInfo instances are stored. Fix issues with
chained unwind areas exposed by the test that were related to this.

The ChainedParent field had the wrong address, because when the chained unwind
info was added, the addresses shifted around. Now we store the pointers to the
structures, which are now allocated from the MC heap.

llvm-svn: 132106