[CTU] Add DisplayCTUProgress analyzer switch

Summary:
With a new switch we may be able to print to stderr if a new TU is being loaded
during CTU.  This is very important for higher level scripts (like CodeChecker)
to be able to parse this output so they can create e.g. a zip file in case of
a Clang crash which contains all the related TU files.

Reviewers: xazax.hun, Szelethus, a_sidorin, george.karpenkov

Subscribers: whisperity, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalho, donat.nagy, dkrupp,

Differential Revision: https://reviews.llvm.org/D55135

llvm-svn: 348594