COFF: Support exception table.

.pdata section contains a list of triplets of function start address,
function end address and its unwind information. Linkers have to
sort section contents by function start address and set the section
address to the file header (so that runtime is able to find it and
do binary search.)

This change seems to resolve all but one remaining test failures in
check{,-clang,-lld} when building the entire stuff with clang-cl and
lld-link.

llvm-svn: 240231