[OpenCL] Add test for __kernel_exec macro (llvm#138287)

The `__kernel_exec` macro is mandated by the OpenCL C specification and
supplied by `opencl-c-base.h`, but it was not covered in any tests.

Author: svenvh

clang/test/CodeGenOpenCL/kernel-attributes.cl

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -fdeclare-opencl-builtins -finclude-default-header -emit-llvm -o - %s | FileCheck %s
 
 typedef unsigned int uint4 __attribute__((ext_vector_type(4)));
 
@@ -11,8 +11,13 @@ kernel __attribute__((vec_type_hint(uint4))) __attribute__((work_group_size_hint
 kernel __attribute__((intel_reqd_sub_group_size(8))) void kernel3(int a) {}
 // CHECK: define {{(dso_local )?}}spir_kernel void @kernel3(i32 {{[^%]*}}%a) {{[^{]+}} !intel_reqd_sub_group_size ![[MD5:[0-9]+]]
 
+__kernel_exec(32, float) void kernel_from_exec(int a) {}
+// CHECK: define {{(dso_local )?}}spir_kernel void @kernel_from_exec(i32 {{[^%]*}}%a) {{[^{]+}} !vec_type_hint ![[MD6:[0-9]+]] !work_group_size_hint ![[MD7:[0-9]+]]
+
 // CHECK: [[MD1]] = !{i32 poison, i32 1}
 // CHECK: [[MD2]] = !{i32 1, i32 2, i32 4}
 // CHECK: [[MD3]] = !{<4 x i32> poison, i32 0}
 // CHECK: [[MD4]] = !{i32 8, i32 16, i32 32}
 // CHECK: [[MD5]] = !{i32 8}
+// CHECK: [[MD6]] = !{float poison, i32 0}
+// CHECK: [[MD7]] = !{i32 32, i32 1, i32 1}