/usr/lib/mono/4.5/Microsoft.Build.dll is in libmono-microsoft-build4.0-cil 4.6.2.7+dfsg-1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/Microsoft.Build/4.0.0.0__b03f5f7f11d50a3a/Microsoft.Build.dll