Allow the creation of human-friendly ASTDumper to arbitrary output stream

Allow the creation of human-friendly ASTDumper to arbitrary output stream

Summary:
`ASTPrinter` allows setting the ouput to any O-Stream, but that printer creates source-code-like syntax (and is also marked with a `FIXME`). The nice, colourful, mostly human-readable `ASTDumper` only works on the standard output, which is not feasible in case a user wants to see the AST of a file through a code navigation/comprehension tool.

This small addition of an overload solves generating a nice colourful AST block for the users of a tool I'm working on, [[ http://github.com/Ericsson/CodeCompass | CodeCompass ]], as opposed to having to duplicate the behaviour of definitions that only exist in the anonymous namespace of implementation TUs related to this module.

Reviewers: alexfh, klimek, rsmith

Reviewed By: alexfh

Subscribers: rnkovacs, dkrupp, gsd, xazax.hun, cfe-commits, #clang

Tags: #clang

Patch by Whisperity!

Differential Revision: https://reviews.llvm.org/D45096

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@329391 91177308-0d34-0410-b5e6-96231b3b80d8