Fix a thinko in how the RemoteiOS Platform looked up files in the SDK & other
platform locations.  We didn't always do an exhaustive search through all the
platform locations, so we would have to read some files out of memory even though
they existed in the exploded shared cache or SDK.

<rdar://problem/18385947>
[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.