[Clang][OpenMP] Emit unsupported directive error (llvm#70233)

Hello!

This PR fixes llvm#63871. Clang should no longer crash and instead emits an
error message.

Below is an example of the new error message:

```
~/dev/fork-llvm-project omp_dispatch_unimpl
❯ ./install/bin/clang -fopenmp  -c -emit-llvm -Xclang -disable-llvm-passes test.c
test.c:6:5: error: cannot compile this OpenMP dispatch directive yet
    6 |     #pragma omp dispatch
      |     ^~~~~~~~~~~~~~~~~~~~
1 error generated.
```

Author: rkchang

clang/lib/CodeGen/CGStmt.cpp

@@ -407,7 +407,7 @@ void CodeGenFunction::EmitStmt(const Stmt *S, ArrayRef<const Attr *> Attrs) {
     EmitOMPInteropDirective(cast<OMPInteropDirective>(*S));
     break;
   case Stmt::OMPDispatchDirectiveClass:
-    llvm_unreachable("Dispatch directive not supported yet.");
+    CGM.ErrorUnsupported(S, "OpenMP dispatch directive");
     break;
   case Stmt::OMPScopeDirectiveClass:
     llvm_unreachable("scope not supported with FE outlining");

clang/test/OpenMP/dispatch_unsupported.c

@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -emit-llvm -fopenmp -disable-llvm-passes %s -verify=expected
+
+// expected-error@+2 {{cannot compile this OpenMP dispatch directive yet}} 
+void a(){
+    #pragma omp dispatch
+    a();
+}