[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@265530 91177308-0d34-0410-b5e6-96231b3b80d8