Use llvm::reverse to make a bunch of loops use foreach. NFC.

In llvm commit r243581, a reverse range adapter was added which allows
us to change code such as

  for (auto I = Fields.rbegin(), E = Fields.rend(); I != E; ++I) {

in to

  for (const FieldDecl *I : llvm::reverse(Fields))

This commit changes a few of the places in clang which are eligible to use
this new adapter.

llvm-svn: 243663