[clangd] context key constructor is constexpr. NFC

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@334824 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/clangd/Context.h b/clangd/Context.h
index 486b123..7e14f86 100644
--- a/clangd/Context.h
+++ b/clangd/Context.h
@@ -43,7 +43,7 @@
   static_assert(!std::is_reference<Type>::value,
                 "Reference arguments to Key<> are not allowed");
 
-  Key() = default;
+  constexpr Key() = default;
 
   Key(Key const &) = delete;
   Key &operator=(Key const &) = delete;