[SYCL] Improve diagnostics for invalid kernel names (#4769)

The use of lambda (which is not forward-declarable) in a SYCL kernel name currently generates the error message: 
error: unnamed lambda ‘(lambda at {{.*}}filename.cpp{{.}’ used

The diagnostic message is very confusing and doesn't help to understand what exactly is wrong. 

To update the message to make it better and more helpful, this patch modifies the error diagnostic when the lambdas (anonymous function object types) are used in SYCL kernel names.

Signed-off-by: Soumi Manna <[email protected]>

Author: smanna12

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -11440,7 +11440,8 @@ def err_invalid_std_type_in_sycl_kernel : Error<"%0 is an invalid kernel name, "
 def err_sycl_kernel_incorrectly_named : Error<
   "%select{%1 should be globally visible"
   "|unscoped enum %1 requires fixed underlying type"
-  "|unnamed lambda %1 used"
+  "|unnamed type %1 is invalid; provide a kernel name, or use "
+  "'-fsycl-unnamed-lambda' to enable unnamed kernel lambdas"
   "}0">;
 
 def err_sycl_kernel_not_function_object

clang/lib/Sema/SemaSYCL.cpp

@@ -3502,8 +3502,7 @@ class SYCLKernelNameTypeVisitor
           if (UnnamedLambdaUsed) {
             S.Diag(KernelInvocationFuncLoc,
                    diag::err_sycl_kernel_incorrectly_named)
-                << /* unnamed lambda used */ 2 << KernelNameType;
-
+                << /* unnamed type is invalid */ 2 << KernelNameType;
             IsInvalid = true;
             return;
           }

clang/test/SemaSYCL/unnamed-kernel.cpp

@@ -137,7 +137,7 @@ struct MyWrapper {
 
     // This errors because const decltype(SomeLambda) != decltype(SomeLambda),
     //  so this is not the unnamed lambda situation.
-    // expected-error@#KernelSingleTask {{unnamed lambda 'const}}
+    // expected-error@#KernelSingleTask {{unnamed type 'const}}
     // expected-note@+3{{in instantiation of function template specialization}}
     q.submit([&](cl::sycl::handler &h) {
       auto SomeLambda = []() {};
@@ -150,7 +150,7 @@ struct MyWrapper {
 int main() {
   cl::sycl::queue q;
 #ifndef __SYCL_UNNAMED_LAMBDA__
-  // expected-error-re@#KernelSingleTask {{unnamed lambda '(lambda at {{.*}}unnamed-kernel.cpp{{.*}}' used}}
+  // expected-error-re@#KernelSingleTask {{unnamed type '(lambda at {{.*}}unnamed-kernel.cpp{{.*}}' is invalid; provide a kernel name, or use '-fsycl-unnamed-lambda' to enable unnamed kernel lambdas}}
   // expected-note@+2{{in instantiation of function template specialization}}
 #endif
   q.submit([&](cl::sycl::handler &h) { h.single_task([] {}); });