[ThinLTO] Double bits of module hash used for renaming

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316996 91177308-0d34-0410-b5e6-96231b3b80d8