[lldb] Support custom LLVM formatting for variables (#91868)

Re-apply https://github.com/llvm/llvm-project/pull/81196, with a fix that handles the 
absence of llvm formatting: 
https://github.com/llvm/llvm-project/pull/91868/commits/3ba650e91eded3543764f37921dcce3b
b47d425f