[clang] Add code completion for C++20 keywords. (PR #107982)

[via cfe-commits]

================
@@ -0,0 +1,35 @@
+const char8_t x = 1;
+
+template<typename T> requires true
+const int y = requires { typename T::type; requires T::value; };
+
+int f(){ co_await 1; }
+
+// RUN: %clang_cc1 -std=c++20 -code-completion-at=%s:1:3 %s | FileCheck --check-prefix=CHECK-TOP-LEVEL %s
+// CHECK-TOP-LEVEL: const
+// CHECK-TOP-LEVEL: consteval
+// CHECK-TOP-LEVEL: constexpr
+// CHECK-TOP-LEVEL: constinit
+
+// RUN: %clang_cc1 -std=c++20 -code-completion-at=%s:1:12 %s | FileCheck --check-prefix=CHECK-TOP-LEVEL %s
+// CHECK-TOP-LEVEL: char8_t
+
+// RUN: %clang-cc1 -std=c++20 -code-completion-at=%s:4:3 %s | FileCheck --check-prefix=CHECK-REQUIRES %s
+// CHECK-REQUIRES: concept
+// CHECK-REQUIRES: const
+// CHECK-REQUIRES: consteval
+// CHECK-REQUIRES: constexpr
+// CHECK-REQUIRES: constinit
+
+// RUN: %clang-cc1 -std=c++20 -code-completion-at=%s:3:27 %s | FileCheck --check-prefix=CHECK-REQUIRES %s
+// CHECK-REQUIRES: requires
+
+// RUN: %clang-cc1 -std=c++20 -code-completion-at=%s:4:20 %s | FileCheck -check-prefix=CHECK-CC1 %s
+// CHECK-CC1-NEXT: COMPLETION: Pattern: [#bool#]requires (<#parameters#>) {
+// CHECK-CC1-NEXT: <#requirements#>
+// CHECK-CC1-NEXT: }
+
+// RUN: %clang-cc1 -std=c++20 -code-completion-at=%s:6:13 %s | FileCheck --check-prefix=CHECK-COAWAIT %s
+// CHECK-COAWAIT: Pattern : co_await <#expression#>
+// CHECK-COAWAIT: Pattern : co_return <#expression#>;
+// CHECK-COAWAIT: Pattern : co_yield <#expression#>
----------------
16bit-ykiko wrote:

fix.

https://github.com/llvm/llvm-project/pull/107982