[OpenMP] Capture global variables in target regions.

[OpenMP] Capture global variables in target regions.

All global variables that are not enclosed in a declare target region
must be captured in the target region as local variables do. Currently,
there is no support for declare target, so this patch adds support for
capturing all the global variables used in a the target region.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249154 91177308-0d34-0410-b5e6-96231b3b80d8