Improve -Winfinite-recursion

Summary: Rewrites -Winfinite-recursion to remove the state dictionary and explore paths in loops - especially infinite loops.  The new check now detects recursion in loop bodies dominated by a recursive call.

Reviewers: rsmith, rtrieu

Reviewed By: rtrieu

Subscribers: lebedev.ri, cfe-commits

Differential Revision: https://reviews.llvm.org/D43737

llvm-svn: 328173