This commit moves the **alpha.security.taint.TaintPropagation** and
**alpha.security.taint.GenericTaint** checkers to the **optin.taint**
optional package.
These checkers were stabilized and improved by recent commits thus
they are ready for production use.
Taint propagation is a a generic modeling feature of the Clang Static
Analyzer which many other checkers depend on. Therefore
GenericTaintChecker is split into a TaintPropagation modeling checker
and a GenericTaint reporting checker.
Checker alpha.security.taint.TaintPropagation now has user documentation for
taint analysis with an example showing external YAML configuration format.
The format of the taint configuration file is now documented under the user
documentation of Clang SA.
Differential Revision: https://reviews.llvm.org/D113251
