[clangd] Cleanup FuzzyFindRequest filtering limit semantics

As discussed during D51860 review, it is better to use `llvm::Optional`
here as it has clear semantics which reflect intended behavior.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D52028

llvm-svn: 342138
diff --git a/clang-tools-extra/clangd/CodeComplete.cpp b/clang-tools-extra/clangd/CodeComplete.cpp
index 1e632dc..64d38dd 100644
--- a/clang-tools-extra/clangd/CodeComplete.cpp
+++ b/clang-tools-extra/clangd/CodeComplete.cpp
@@ -1375,7 +1375,7 @@
     // Build the query.
     FuzzyFindRequest Req;
     if (Opts.Limit)
-      Req.MaxCandidateCount = Opts.Limit;
+      Req.Limit = Opts.Limit;
     Req.Query = Filter->pattern();
     Req.RestrictForCodeCompletion = true;
     Req.Scopes = QueryScopes;