[OpenMP] Generate better diagnostics for cancel and cancellation point

checkNestingOfRegions uses CancelRegion to determine whether cancel and
cancellation point are valid in the given nesting. This leads to unuseful
diagnostics if CancelRegion is invalid. The given test case has produced:
  region cannot be closely nested inside 'parallel' region

As a solution, introduce checkCancelRegion and call it first to get the
expected error:
  one of 'for', 'parallel', 'sections' or 'taskgroup' is expected

Differential Revision: https://reviews.llvm.org/D30135

llvm-svn: 295808