[OPENMP] Allow runtime insert its own code inside OpenMP regions.

Solution unifies interface of RegionCodeGenTy type to allow insert
runtime-specific code before/after main codegen action defined in
CGStmtOpenMP.cpp file. Runtime should not define its own RegionCodeGenTy
for general OpenMP directives, but must be allowed to insert its own
  (required) code to support target specific codegen.

llvm-svn: 264569