Add an llvm::cl::opt::operator=(T &&Val) (#147502)

Add an llvm::cl::opt::operator= that takes an rvalue reference of value,
to avoid an unecessary copy for types with memory allocation (string,
vector, etc).

Author: tomnatan30

llvm/include/llvm/Support/CommandLine.h

@@ -1496,6 +1496,12 @@ class opt
     return this->getValue();
   }
 
+  template <class T> DataType &operator=(T &&Val) {
+    this->getValue() = std::forward<T>(Val);
+    Callback(this->getValue());
+    return this->getValue();
+  }
+
   template <class... Mods>
   explicit opt(const Mods &... Ms)
       : Option(llvm::cl::Optional, NotHidden), Parser(*this) {