[PECOFF] Set ordinal to alias atoms

Atoms are ordered in the output file by ordinal. File has file ordinal,
and atom has atom ordinal which is unique within the file.
No two atoms should have the same combination of ordinals.

However that contract was not satisifed for alias atoms. Alias atom
is defined by /alternatename:sym1=sym2. In this case sym1 is defined
as an alias for sym2. sym1 always got ordinal 0.

As a result LLD failed with an assertion failure.

This patch assigns ordinal to alias atoms.

llvm-svn: 218158