[OpenMP] Cast captures by copy when passed to fork call so that they are compatible to what the runtime library expects.

Summary:
This patch fixes an issue detected when firstprivate variables are passed to an OpenMP outlined function vararg list. Currently they are not compatible with what the runtime library expects causing malfunction in some targets.

This patch fixes the issue by moving the casting logic already in place for offloading to the common code that creates the outline function and arguments and updates the regression tests accordingly.

Reviewers: hfinkel, arpith-jacob, carlo.bertolli, kkwli0, ABataev

Subscribers: cfe-commits, caomhin

Differential Revision: http://reviews.llvm.org/D21150

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272900 91177308-0d34-0410-b5e6-96231b3b80d8