[analyzer] Make CloneDetector recognize different variable patterns.

CloneDetector should be able to detect clones with renamed variables.
However, if variables are referenced multiple times around the code sample,
the usage patterns need to be recognized.

For example, (x < y ? y : x) and (y < x ? y : x) are no longer clones,
however (a < b ? b : a) is still a clone of the former.

Variable patterns are computed and compared during a separate filtering pass.

Patch by Raphael Isemann!

Differential Revision: https://reviews.llvm.org/D22982

llvm-svn: 277757