[ThinLTO] Double bits of module hash used for renaming

Summary:
Use 64 instead of 32 bits of the module hash as the suffix when renaming
after promotion to reduce the likelihood of a collision (which we
observed in a binary when using 32 bits).

Reviewers: pcc

Subscribers: llvm-commits, inglorion

Differential Revision: https://reviews.llvm.org/D39443

llvm-svn: 316996