[lldb][NFC] Move LLVM RTTI implementation from enum to static ID variable

Summary:
swift-lldb currently has to patch the ExpressionKind enum to add support for Swift expressions. If we implement LLVM's RTTI
with a static ID variable instead of a centralised enum we can drop that patch.

Reviewers: labath, davide

Reviewed By: labath

Subscribers: JDevlieghere, lldb-commits

Tags: #upstreaming_lldb_s_downstream_patches, #lldb

Differential Revision: https://reviews.llvm.org/D70070