Replace 'mkdir' shell invocation by native function call.

Summary: Now that Host provide a MakeDirectory function, we can use it instead of relying on command line tool to create a directory.

CC: lldb-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2356

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@196801 91177308-0d34-0410-b5e6-96231b3b80d8