FunctionImport: add a progressive heuristic to limit importing too deep in the callgraph

The current function importer will walk the callgraph, importing
transitively any callee that is below the threshold. This can
lead to import very deep which is costly in compile time and not
necessarily beneficial as most of the inline would happen in
imported function and not necessarilly in user code.

The actual factor has been carefully chosen by flipping a coin ;)
Some tuning need to be done (just at the existing limiting threshold).

Reviewers: tejohnson

Differential Revision: http://reviews.llvm.org/D17082

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 260466