[clang] Allow CompilerInvocation to change CASOptions

Add APIs to change CASOptions inside CompilerInvocation. This is needed
to make copies of CompilerInvocations but share the underlying CAS
storage and avoid making more CAS storages that points to the same
location.

clang/include/clang/Frontend/CompilerInvocation.h

@@ -304,6 +304,10 @@ class CompilerInvocation : public CompilerInvocationBase {
     return PPOpts;
   }
   std::shared_ptr<LangOptions> getLangOptsPtr() { return LangOpts; }
+  std::shared_ptr<CASOptions> getCASOptsPtr() { return CASOpts; }
+  void setCASOption(std::shared_ptr<CASOptions> CASOpts) {
+    this->CASOpts = CASOpts;
+  }
   /// @}
 
   /// Create a compiler invocation from a list of input options.