[OpenMP] Map clause codegeneration.

Summary:
Implement codegen for the map clause. All the new list items in 4.5 specification are supported. 

Fix bug in the generation of array sections that was exposed by some of the map clause tests: for pointer types the offsets have to be calculated from the pointee not the pointer.

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

Subscribers: ABataev, cfe-commits, caomhin, fraggamuffin

Differential Revision: http://reviews.llvm.org/D16749

llvm-svn: 267808