Add ConstString::IsNull() to tell between null vs. empty strings and fix usage in Mangled::GetDemangledName()
Summary: IsEmpty() and operator bool() == false have equal semantics. Usage in Mangled::GetDemangledName() was incorrect. What it actually wants is a check for null-string. Split this off of D50071 and added a test to clarify usage.
Reviewers: labath, jingham
Subscribers: erik.pilkington, lldb-commits
Differential Revision: https://reviews.llvm.org/D50327