ELF: Move build id computation to Writer. NFCI.

With partitions, each partition should have the same build id. This means
that the build id needs to be only computed once, otherwise we will end up
with different build ids in each partition as a result of the file contents
changing. This change moves the computation of the build id into Writer so
that it only happens once.

Differential Revision: https://reviews.llvm.org/D60342

llvm-svn: 358536