[CrossTU] Fix problem with CrossTU AST load limit and progress messages.

Summary:
Number of loaded ASTs is to be incremented only if the AST was really loaded
but not if it was returned from cache. At the same place the message about
a loaded AST is displayed.

Reviewers: martong, gamesh411

Reviewed By: martong

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66054

llvm-svn: 368545