[analyzer][CrossTU] Lower CTUImportThreshold default value

Summary:
The default value of 100 makes the analysis slow. Projects of considerable
size can take more time to finish than it is practical. The new default
setting of 8 is based on the analysis of LLVM itself. With the old default
value of 100 the analysis time was over a magnitude slower. Thresholding the
load of ASTUnits is to be extended in the future with a more fine-tuneable
solution that accomodates to the specifics of the project analyzed.

Reviewers: martong, balazske, Szelethus

Subscribers: whisperity, xazax.hun, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, Charusso, steakhal, ASDenysPetrov, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82561