[LSan] Export __lsan_init

Summary:
lsan_preinit.cc is meant to be linked into executable and calls
lsan_init from .preinit_array section. But if liblsan is a shared library,
then this doesn't work, because the symbol is not exported. This patch fixes
that. The counterparts like asan_init or __tsan_init already do have
SANITIZER_INTERFACE_ATTRIBUTE.

Committing on behalf of jakubjelinek.

Reviewers: alekseyshl, kcc

Subscribers: llvm-commits

Differential revision: https://reviews.llvm.org/D39889

llvm-svn: 318349