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