Move Host::GetLLDBPath to HostInfo.

This continues the effort to get Host code moved over to HostInfo,
and removes many more instances of preprocessor defines along the
way.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@216195 91177308-0d34-0410-b5e6-96231b3b80d8