Clarify builtins are also available in C.

Author: AlexVlx

clang/docs/LanguageExtensions.rst

@@ -5033,6 +5033,35 @@ target specific type that behaves as if its C++ definition was the following:
     operator bool() const noexcept;
   };
 
+The builtins can be used in C as well, wherein the
+``__amdgpu_feature_predicate_t`` type behaves as an opaque, forward declared
+type with conditional automated conversion to ``_Bool`` when used as the
+predicate argument to a control structure:
+
+.. code-block:: c
+
+  struct __amdgpu_feature_predicate_t ret();     // Error
+  void arg(struct __amdgpu_feature_predicate_t); // Error
+  void local() {
+    struct __amdgpu_feature_predicate_t x;       // Error
+    struct __amdgpu_feature_predicate_t y =
+        __builtin_amdgcn_processor_is("gfx900"); // Error
+  }
+  void valid_use() {
+    _Bool x = (_Bool)__builtin_amdgcn_processor_is("gfx900"); // OK
+    if (__builtin_amdgcn_processor_is("gfx900"))       // Implicit cast to _Bool
+      return;
+    for (; __builtin_amdgcn_processor_is("gfx900");)   // Implicit cast to _Bool
+      break;
+    while (__builtin_amdgcn_processor_is("gfx900"))    // Implicit cast to _Bool
+      break;
+    do {
+      break;
+    } while (__builtin_amdgcn_processor_is("gfx900")); // Implicit cast to _Bool
+
+    __builtin_amdgcn_processor_is("gfx900") ? x : !x;
+  }
+
 The boolean interpretation of the predicate values returned by the builtins:
 
 * indicates whether the current target matches the argument; the argument MUST