Add ConstString::IsNull() to tell between null vs. empty strings and fix usage...

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

llvm-svn: 339014