cleanup the MappedFile API and comments. This removes and updates

cleanup the MappedFile API and comments.  This removes and updates
tons of out of date comments (really nothing throws here!) and fixes
some other fairly glaring issues: "size" used to return the size of
the file *and* change it, depending on how you called it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49009 91177308-0d34-0410-b5e6-96231b3b80d8