[OPENMP]Improve detection of number of teams, threads in target

[OPENMP]Improve detection of number of teams, threads in target
regions.

Added more complex analysis for number of teams and number of threads in
the target regions, also merged related common code between CGOpenMPRuntime
and CGOpenMPRuntimeNVPTX classes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@358126 91177308-0d34-0410-b5e6-96231b3b80d8