[PECOFF] Sort export table properly.

Export table entries need to be sorted in ASCII-betical order,
so that the loader can find an entry for a function by binary search.

We sorted the entries by its mangled names. That can be different
from their exported names. As a result, LLD produces incorrect export
table, from which the loader complains that a function that actually
exists in a DLL cannot be found.

This patch fixes that issue.

llvm-svn: 222452