[OPENMP50]Perform data mapping analysis only for explicitly mapped data.

Summary:
According to OpenMP 5.0, the restrictions for mapping of overlapped data
apply only for explicitly mapped data, there is no restriction for
implicitly mapped data just like in OpenMP 4.5.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, sstefan1, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83398