Add indented raw_ostream class

Class simplifies keeping track of the indentation while emitting. For every new line the current indentation is simply prefixed (if not at start of line, then it just emits as normal). Add a simple Region helper that makes it easy to have the C++ scope match the emitted scope.

Use this in op doc generator and rewrite generator.

Differential Revision: https://reviews.llvm.org/D84107