Add macro LLVM_ATTRIBUTE_REINITIALIZES

[MaskRay]

Summary

This marks legitimate use-after-move (e.g. Found.clear() in rC342925)
which would otherwise be caught by bugprone-use-after-move.

bugprone-use-after-move recognizes this attribute after rCTE339571.

llvm/trunk/include/llvm/Support/Compiler.h

 // error when __has_cpp_attribute is given a scoped attribute in C mode.
 #define LLVM_NODISCARD
 #elif __has_cpp_attribute(clang::warn_unused_result)
 #define LLVM_NODISCARD [[clang::warn_unused_result]]
 #else
 #define LLVM_NODISCARD
 #endif
 
+// Indicate that a non-static, non-const C++ member function reinitializes
+// the entire object to a known state, independent of the previous state of
+// the object.
+//
+// The clang-tidy check bugprone-use-after-move recognizes this attribute as a
+// marker that a moved-from object has left the indeterminate state and can be
+// reused.
+#if __has_cpp_attribute(clang::reinitializes)
+#define LLVM_ATTRIBUTE_REINITIALIZES [[clang::reinitializes]]
+#else
+#define LLVM_ATTRIBUTE_REINITIALIZES
+#endif
+
 // Some compilers warn about unused functions. When a function is sometimes
 // used or not depending on build settings (e.g. a function only called from
 // within "assert"), this attribute can be used to suppress such warnings.
 //
 // However, it shouldn't be used for unused *variables*, as those have a much
 // more portable solution:
 //   (void)unused_var_name;
 // Prefer cast-to-void wherever it is sufficient.