[OpenMP] Prevent nesting of target constructs within target code execution regions.

[OpenMP] Prevent nesting of target constructs within target code execution regions.

Summary:
This patch enhances Sema to check for the following restriction:

OpenMP 4.5 [2.17 Nesting of Regions]
If a target, target update, target data, target enter data, or
target exit data construct is encountered during execution of a
target region, the behavior is unspecified.

Reviewers: ABataev

Differential Revision: http://reviews.llvm.org/D16758

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@259366 91177308-0d34-0410-b5e6-96231b3b80d8