Mangled: Fix an 'unused variable' warning on GNU/Linux

Platforms which don't use LLDB's built-in demangler don't use the
'mangled_length' variable.  Instead, replace it's only use by an
expression it is equivalent to.

Differential Revision: http://reviews.llvm.org/D4625

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@213681 91177308-0d34-0410-b5e6-96231b3b80d8