Add ConstString test FromMidOfBufferStringRef

Summary: It was not immediately clear to me whether or not non-null-terminated StringRef's are supported in ConstString and/or the counterpart mechanism. From this test it seems to be fine. Maybe useful to keep?

Reviewers: labath

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D50334

llvm-svn: 339292