[ELF] Implement demangle.

This adds functionality in the GNU flavor to demangle symbols when
undefined symbols are displayed to the user.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220184 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Add demangle test to XFAIL

This test is failing for some reason on freebsd.

Not sure why it should fail, need to investigate.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220185 91177308-0d34-0410-b5e6-96231b3b80d8