[DependencyCollector] Make maybeAddDependency virtual (NFC)

Make DependencyCollector::maybeAddDependency, just like its other
methods, which I made virtual a while ago. The motivation for this
change is still the LLDB reproducer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367271 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Frontend/Utils.h

@@ -99,11 +99,11 @@ public:
   /// Return true if system files should be passed to sawDependency().
   virtual bool needSystemDependencies() { return false; }
 
-  // implementation detail
   /// Add a dependency \p Filename if it has not been seen before and
   /// sawDependency() returns true.
-  void maybeAddDependency(StringRef Filename, bool FromModule, bool IsSystem,
-                          bool IsModuleFile, bool IsMissing);
+  virtual void maybeAddDependency(StringRef Filename, bool FromModule,
+                                  bool IsSystem, bool IsModuleFile,
+                                  bool IsMissing);
 
 protected:
   /// Return true if the filename was added to the list of dependencies, false