Add a feature for debugging library dependency cycles, -why option. This

Add a feature for debugging library dependency cycles, -why option. This
implies -flat and will produce a list of all the symbols for each library
that another library depends on. Run the output through c++filt for
better readability. Also, don't generate a temporary file for storing the
dependent library names. Perl can handle it in a %hash.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29273 91177308-0d34-0410-b5e6-96231b3b80d8