Add a funciton to get the segment name of a section.

Add a funciton to get the segment name of a section.

On MachO, sections also have segment names. When a tool looking at a .o file
prints a segment name, this is what they mean. In reality, a .o has only one,
anonymous, segment.

This patch adds a MachO only function to fetch that segment name. I named it
getSectionFinalSegmentName since the main use for the name seems to be informing
the linker with segment this section should go to.

The patch also changes MachOObjectFile::getSectionName to return just the
section name instead of computing SegmentName,SectionName.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170095 91177308-0d34-0410-b5e6-96231b3b80d8