Speculative fix for bots after 84e8257

I'm not seeing an error locally, but many bots are having a problem with
the call to `hasOptionalValue`. Try to fix it.

Author: dexonsmith

clang/include/clang/Basic/FileEntry.h

@@ -148,7 +148,7 @@ class FileEntryRef {
 
   // Private constructor for use by OptionalStorage.
   FileEntryRef(optional_none_tag) : ME(nullptr) {}
-  bool hasOptionalValue() const { return ME; }
+  constexpr bool hasOptionalValue() const { return ME; }
 
   const MapEntry *ME;
 };