[lldb] Don't persist the LINKEDIT slide in the indirect symbol offset

The current code increment the indirect symbol offset with the LINKEDIT
slide every time ObjectFileMachO::ParseSymtab is called.

This resulted in a crash when calling add-dsym which causes us to
potentially re-parse the original binary's symbol table. There's a
separate question about whether we should re-parse the symbol table at
all which was fixed by D114288. Regardless, copying the load command is
cheap enough that this is still the right thing to do.

rdar://72337717

Differential revision: https://reviews.llvm.org/D122349