[analyzer] NFC: GenericTaintChecker: Revise rule specification mechanisms.

Provide a more powerful and at the same time more readable way of specifying
taint propagation rules for known functions within the checker.

Now it should be possible to specify an unlimited amount of source and
destination parameters for taint propagation.

No functional change intended just yet.

Patch by Gábor Borsik!

Differential Revision: https://reviews.llvm.org/D55734

llvm-svn: 352572