[clangd] Fix a stale comment, NFC.

llvm-svn: 348133
diff --git a/clang-tools-extra/clangd/index/SymbolCollector.cpp b/clang-tools-extra/clangd/index/SymbolCollector.cpp
index 52a2069..b78bc5f 100644
--- a/clang-tools-extra/clangd/index/SymbolCollector.cpp
+++ b/clang-tools-extra/clangd/index/SymbolCollector.cpp
@@ -260,7 +260,7 @@
 }
 
 bool SymbolCollector::shouldCollectSymbol(const NamedDecl &ND,
-                                          ASTContext &ASTCtx,
+                                          const ASTContext &ASTCtx,
                                           const Options &Opts) {
   if (ND.isImplicit())
     return false;