[OPENMP] Generate implicit map|firstprivate clauses for target-based

[OPENMP] Generate implicit map|firstprivate clauses for target-based
directives.

If the variable is used in the target-based region but is not found in
any private|mapping clause, then generate implicit firstprivate|map
clauses for these implicitly mapped variables.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314205 91177308-0d34-0410-b5e6-96231b3b80d8