[DWARF parser] Teach DIContext to fetch short (non-linkage) function names for a given address.

Change --functions option in llvm-symbolizer tool to accept
values "none", "short" or "linkage". Update the tests and docs
accordingly.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209050 91177308-0d34-0410-b5e6-96231b3b80d8