[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

lldb/source/Expression/UserExpression.cpp

@@ -46,13 +46,14 @@
 
 using namespace lldb_private;
 
+char UserExpression::ID;
+
 UserExpression::UserExpression(ExecutionContextScope &exe_scope,
                                llvm::StringRef expr, llvm::StringRef prefix,
                                lldb::LanguageType language,
                                ResultType desired_type,
-                               const EvaluateExpressionOptions &options,
-                               ExpressionKind kind)
-    : Expression(exe_scope, kind), m_expr_text(expr), m_expr_prefix(prefix),
+                               const EvaluateExpressionOptions &options)
+    : Expression(exe_scope), m_expr_text(expr), m_expr_prefix(prefix),
       m_language(language), m_desired_type(desired_type), m_options(options) {}
 
 UserExpression::~UserExpression() {}