[ThinLTO] Avoid archive member collisions with old API

Summary:
ld64 on OSX uses the old ThinLTOCodegenerator API. When two modules have the same name in an archive (valid archive), a name collision happens for the modules' buffer identifiers.
This PR resolves this, by suffixing the module name with an increasing number such that the identifiers are guaranteed to be unique.

For a similar fix in LLD, see https://reviews.llvm.org/D25495

Reviewers: mehdi_amini, tejohnson

Reviewed By: mehdi_amini

Subscribers: inglorion, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D37961

llvm-svn: 313488