[msan] Put msan constructor in a comdat.

MSan adds a constructor to each translation unit that calls
__msan_init, and does nothing else. The idea is to run __msan_init
before any instrumented code. This results in multiple constructors
and multiple .init_array entries in the final binary, one per
translation unit. This is absolutely unnecessary; one would be
enough.

This change moves the constructors to a comdat group in order to drop
the extra ones.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260632 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed