[Clangd] Fix ambiguous use of 'Range'

Summary:
The changes in D155173 caused build failures on some configruations due
to the use of `Range` which conflicts with other definitions. Explicitly
use `clangd::Range`.

clang-tools-extra/clangd/ClangdServer.h

@@ -353,7 +353,7 @@ class ClangdServer {
 
   // Ref to the clangd::Diag.
   struct DiagRef {
-    Range Range;
+    clangd::Range Range;
     std::string Message;
     bool operator==(const DiagRef &Other) const {
       return std::tie(Range, Message) == std::tie(Other.Range, Other.Message);