[OPENMP] Support for '#pragma omp taskgroup' directive.

Added parsing, sema analysis and codegen for '#pragma omp taskgroup' directive (OpenMP 4.0).
The code for directive is generated the following way:
#pragma omp taskgroup
<body>

void __kmpc_taskgroup(<loc>, thread_id);
<body>
void __kmpc_end_taskgroup(<loc>, thread_id);

llvm-svn: 240011
diff --git a/clang/lib/Basic/OpenMPKinds.cpp b/clang/lib/Basic/OpenMPKinds.cpp
index b83a069..b2798b7 100644
--- a/clang/lib/Basic/OpenMPKinds.cpp
+++ b/clang/lib/Basic/OpenMPKinds.cpp
@@ -332,6 +332,7 @@
   case OMPD_taskyield:
   case OMPD_barrier:
   case OMPD_taskwait:
+  case OMPD_taskgroup:
   case OMPD_ordered:
     break;
   }