Port SelectionTree from clangd to clice (#166)

include/AST/SourceCode.h

@@ -1,8 +1,39 @@
 #pragma once
 
-#include "SourceLocation.h"
+#include <tuple>
 #include "clang/Lex/Token.h"
-#include "llvm/ADT/FunctionExtras.h"
+#include "clang/Basic/SourceLocation.h"
+
+namespace std {
+
+template <>
+struct tuple_size<clang::SourceRange> : std::integral_constant<std::size_t, 2> {};
+
+template <>
+struct tuple_element<0, clang::SourceRange> {
+    using type = clang::SourceLocation;
+};
+
+template <>
+struct tuple_element<1, clang::SourceRange> {
+    using type = clang::SourceLocation;
+};
+
+}  // namespace std
+
+namespace clang {
+
+/// Through ADL, make `clang::SourceRange` could be destructured.
+template <std::size_t I>
+clang::SourceLocation get(clang::SourceRange range) {
+    if constexpr(I == 0) {
+        return range.getBegin();
+    } else {
+        return range.getEnd();
+    }
+}
+
+}  // namespace clang
 
 namespace clice {