Fix GetDisplayName when only a demangled name is available

Summary:
GetDisplayDemangledName will already return a ConstString() when
there is neither a mangled name or a demangled name, so we don't need to special
case here. This will fix GetDisplayName in cases where m_mangled contains
only a demangled name and not a mangled name.

Reviewers: clayborg, granata.enrico, sas

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D25201

llvm-svn: 283491