[lldb][NFCI] Remove unneeded use of ConstString in ASTResultSynthesizer

2/3 of the ConstStrings in this class were just to be able to log
something. Putting something in the StringPool just to log it doesn't
make a lot of sense, so let's remove them.

The remaining use is for `RegisterPersistentDecl` which is fine for now.

Differential Revision: https://reviews.llvm.org/D153905