[Layout] Dont set ordinals to Files by default.

This change removes code in various places which was setting the File Ordinals.
This is because the file ordinals are assigned by the way files are resolved.

There was no other way than making the getNextFileAndOrdinal be set const and
change the _nextOrdinal to mutable.

There are so many places in code, that you would need to cleanup to make
LinkingContext non-const!

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192280 91177308-0d34-0410-b5e6-96231b3b80d8