[analyzer] Support for naive cross translation unit analysis

The aim of this patch is to be minimal to enable incremental development of
the feature on the top of the tree. This patch should be an NFC when the
feature is turned off. It is turned off by default and still considered as
experimental.

Technical details are available in the EuroLLVM Talk: 
http://llvm.org/devmtg/2017-03//2017/02/20/accepted-sessions.html#7

Note that the initial prototype was done by A. Sidorin et al.: http://lists.llvm.org/pipermail/cfe-dev/2015-October/045730.html

Contributions to the measurements and the new version of the code: Peter Szecsi, Zoltan Gera, Daniel Krupp, Kareem Khazem.

Differential Revision: https://reviews.llvm.org/D30691

llvm-svn: 326323