Backport r339704 to 7.0 for PR38598

> Author: abataev
> Date: Tue Aug 14 11:31:20 2018
> New Revision: 339704
>
> URL: http://llvm.org/viewvc/llvm-project?rev=339704&view=rev
> Log:
> [OPENMP] Fix processing of declare target construct.
>
> The attribute marked as inheritable since OpenMP 5.0 supports it +
> additional fixes to support new functionality.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_70@339998 91177308-0d34-0410-b5e6-96231b3b80d8