Change the LLVM_ATTRIBUTE_DEPRECATED macro to use C++14 attribute.

[csigg]

Summary

C++14 attributes are superior because they can be applied to functions with inline definition and the syntax is cleaner.

I intend to convert all uses and then remove the macro.

One issue that might hold back switching uses to C++14 attributes is that
clang-format does not put long attributes on separate lines and formatted code will look like:

template <typename T>
[[deprecated("blah blah")]] void 
    foooooooooooooooooooooooooooo() {
  ...
}

Putting long attributes on a separate line would be prettier.
See https://stackoverflow.com/questions/45740466/clang-format-setting-to-control-c-attributes

AttributeMacros probably won't help because it can't match the custom message.
https://clang.llvm.org/docs/ClangFormatStyleOptions.html

[rriddle]

LGTM from me, but can you send a post to llvm-dev to make sure there is general consensus?

[MaskRay]

Thanks!

llvm/include/llvm/Support/Compiler.h

 /// pedantic diagnostics.
 #ifdef __GNUC__
 #define LLVM_EXTENSION __extension__
 #else
 #define LLVM_EXTENSION
 #endif
 
 // LLVM_ATTRIBUTE_DEPRECATED(decl, "message")
-#if __has_feature(attribute_deprecated_with_message)
-# define LLVM_ATTRIBUTE_DEPRECATED(decl, message) \
-  decl __attribute__((deprecated(message)))
-#elif defined(__GNUC__)
-# define LLVM_ATTRIBUTE_DEPRECATED(decl, message) \
-  decl __attribute__((deprecated))
-#elif defined(_MSC_VER)
-# define LLVM_ATTRIBUTE_DEPRECATED(decl, message) \
-  __declspec(deprecated(message)) decl
-#else
-# define LLVM_ATTRIBUTE_DEPRECATED(decl, message) \
-  decl
-#endif
+// This macro will be removed.
+// Use C++14's attribute instead: [[deprecated("message")]]
+#define LLVM_ATTRIBUTE_DEPRECATED(decl, message) [[deprecated(message)]] decl
 
 /// LLVM_BUILTIN_UNREACHABLE - On compilers which support it, expands
 /// to an expression which states that it is undefined behavior for the
 /// compiler to reach this point.  Otherwise is not defined.
 #if __has_builtin(__builtin_unreachable) || LLVM_GNUC_PREREQ(4, 5, 0)
 # define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
 #elif defined(_MSC_VER)
 # define LLVM_BUILTIN_UNREACHABLE __assume(false)