[OPENMP]Make lambda mapping follow reqs for PTR_AND_OBJ mapping.

Summary:
The base pointer for the lambda mapping must point to the lambda capture
placement and pointer must point to the captured variable itself. Patch
fixes this problem.

Reviewers: gtbercea

Subscribers: guansong, openmp-commits, kkwli0, caomhin

Differential Revision: https://reviews.llvm.org/D54260

llvm-svn: 346407