[lldb][NFCI] Remove unneeded ConstString conversions

ConstString can be implicitly converted into a llvm::StringRef. This is
very useful in many places, but it also hides places where we are
creating a ConstString only to use it as a StringRef for the entire
lifespan of the ConstString object.

I locally removed the implicit conversion and found some of the places we
were doing this.

Differential Revision: https://reviews.llvm.org/D159237