[preprocessor] Add an opportunity to retain excluded conditional blocks

It is handy for clang tooling, for instance, in source to source transformation.

Reviewers: vpykhtin (Valery Pykhtin), erichkeane (Erich Keane)

Subscribers: rsmith (Richard Smith), akyrtzi (Argyrios Kyrtzidis)

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66597

llvm-svn: 370123