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

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

Reviewed By: rriddle, MaskRay

Differential Revision: https://reviews.llvm.org/D94219

llvm/include/llvm/Support/Compiler.h

@@ -314,19 +314,9 @@
 #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