Rename Action::begin() to Action::input_begin().

Rename Action::begin() to Action::input_begin().

Also introduce inputs() that reutnrs an llvm::iterator_range.
Iterating over A->inputs() is much less mysterious than
iterating over *A.  No intended behavior change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261674 91177308-0d34-0410-b5e6-96231b3b80d8