[clang] [clang][deps] Expand `DependencyActionController` APIs (PR #184917)

[Jan Svoboda via cfe-commits]

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@@ -75,8 +75,55 @@ class DependencyActionController {
 public:
   virtual ~DependencyActionController();
 
+  /// Creates a thread-safe copy of the controller.
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jansvoboda11 wrote:

I reworded this to hopefully make this clearer.

https://github.com/llvm/llvm-project/pull/184917