[analyzer] StackFrameContext: Add NodeBuilderContext::blockCount() to its profile

Summary:
It allows discriminating between stack frames of the same call that is
called multiple times in a loop.

Thanks to Artem Dergachev for the great idea!

Reviewed By: NoQ

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65587

llvm-svn: 367608