[lldb] Remove the user-defined copy-ctor in ConstString

ConstString is essentially trivially copyable yet it has a user defined
copy constructor that copies its one member pointer. Remove it so it
qualifies as trivial in the eyes of the compiler.

This also fixes two unused variable warnings now that the compiler knows
that the constructor has no side-effects.

Differential revision: https://reviews.llvm.org/D84440