Highlight new C++20 Concepts keywords ("concept" and "requires")

[HighCommander4]

In the following code, which compiles with clang trunk in -std=c++2a mode:

template <typename T>
concept Fooable = requires (T t) { t.foo(); };

template <typename T> requires Fooable<T>
void bar(T t) {
  t.foo();
}

the concept keyword and both instances of the requires keyword should be highlighted as keywords.

[HighCommander4]

Looks like this is covered (along with other C++20 keywords) in #396.

[jeff-hykin]

Thanks for looking through the other issues