Fix unaligned memory access when reading INPUT_FILE_OFFSETS data

Summary: The blob data is unaligned, so we also should read it as such. Should fix the random failures with the sanitizer builds.

Reviewers: rsmith, v.g.vassilev

Reviewed By: v.g.vassilev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D47247

llvm-svn: 333069