[CTU] Add DisplayCTUProgress analyzer switch

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@348594 91177308-0d34-0410-b5e6-96231b3b80d8