[Clang] Set -mlinker-version explicitly in the lto test

This test assumes that the driver will set -object_path_lto linker
flag and it requests the platform linker with -fuse-ld=. When lld is
used as the host linker, the host linker version is unset and so
Clang won't set -object_path_lto since that flag is set conditionally
only when linker version is at least 116. We set the linker version
explicitly to make sure that -object_path_lto is set. That approach
is already used elsewhere in the test.

Differential Revision: https://reviews.llvm.org/D122110