[OpenMP]Skip generating this[:1] map info for non-member variable.

My change of D14093 is only fixed problem for "pragma target data".

The problem still here for "pragma target"
what I am missing is:
When processing "pragma target data", the VD is passed when call to
emitCombinedEntry, so check VD is null as map for this pointer.

But when processing "pragma target" the VD is passed as nullptr, so
check VD is null is not working.

To fix this I add a new parameter IsMapThis. During the call to
emitCombinedEntry passes true if it is capturing this pointer and use
that instead check of "!VD".

Differential Revision: https://reviews.llvm.org/D146000