[OPENMP] Parsing and Sema support for 'omp declare target' directive

Add parsing, sema analysis for 'declare target' construct for OpenMP 4.0
(4.5 support will be added in separate patch).

The declare target directive specifies that variables, functions (C, C++
and Fortran), and subroutines (Fortran) are mapped to a device. The declare
target directive is a declarative directive. In Clang declare target is
implemented as implicit attribute for the declaration.

The syntax of the declare target directive is as follows:

 #pragma omp declare target
 declarations-definition-seq
 #pragma omp end declare target

Based on patch from Michael Wong http://reviews.llvm.org/D15321

llvm-svn: 265530
diff --git a/clang/lib/Basic/OpenMPKinds.cpp b/clang/lib/Basic/OpenMPKinds.cpp
index fa40d01..07a410f 100644
--- a/clang/lib/Basic/OpenMPKinds.cpp
+++ b/clang/lib/Basic/OpenMPKinds.cpp
@@ -547,6 +547,8 @@
       break;
     }
     break;
+  case OMPD_declare_target:
+  case OMPD_end_declare_target:
   case OMPD_unknown:
   case OMPD_threadprivate:
   case OMPD_section: