[OpenMP] Diagnose undeclared variables on declare target clause

Clang asserts on undeclared variables on the to or link clause in the declare
target directive. The patch is to properly diagnose the error.

// foo1 and foo2 are not declared
#pragma omp declare target to(foo1)
#pragma omp declare target link(foo2)

Differential Revision: https://reviews.llvm.org/D40588

llvm-svn: 319458