[OPENMP] Small comment and reformatting fixes.

Fix post-commit review comments by Carlo Bertolli
for commit r209660 - OMP collapse clause.

Re-formatted TransformOMP* functions in TreeTransform.h with clang-format.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210169 91177308-0d34-0410-b5e6-96231b3b80d8