[analyzer] Use faster hashing (MD5) in CloneDetector.

This replaces the old approach of fingerprinting every AST node into a string,
which avoided collisions and was simple to implement, but turned out to be
extremely ineffective with respect to both performance and memory.

The collisions are now dealt with in a separate pass, which no longer causes
performance problems because collisions are rare.

Patch by Raphael Isemann!

Differential Revision: https://reviews.llvm.org/D22515

llvm-svn: 279378