[lldb] Scaffolding for extended variable support.
This patch handles most of the scaffolding for extended variable support that isn't directly tied to functional changes. This patch will be used by one following patch that actually modifies the lldb_private::StackFrame API to allow us to fetch extended variables.
There were a couple important/interesting decisions made in this patch that should be noted:
- Any value type may be 'extended', which is why it's a mask applied over the top of another value type.
- When printing frame variables with `fr v`, default to showing extended variables.
This new value type mask makes some of the ValueType handling more interesting, but since nothing generates objects with this mask until the next patch, we can land the concept in this patch in some amount of isolation.
stack-info: PR: https://github.com/llvm/llvm-project/pull/181500, branch: users/bzcheeseman/stack/8
