[OPENMP][NVPTX]Improve emission of the globalized variables for

[OPENMP][NVPTX]Improve emission of the globalized variables for
target/teams/distribute regions.

Target/teams/distribute regions exist for all the time the kernel is
executed. Thus, if the variable is declared in their context and then
escape it, we can allocate global memory statically instead of
allocating it dynamically.
Patch captures all the globalized variables in target/teams/distribute
contexts, merges them into the records, one per each target region.
Those records are then joined into the union, one per compilation unit
(to save the global memory). Those units are organized into
2 x dimensional arrays, where the first dimension is
the number of blocks per SM and the second one is the number of SMs.
Runtime functions manage this global memory space between the executing
teams.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@345978 91177308-0d34-0410-b5e6-96231b3b80d8