[analyzer] Add yaml parser to GenericTaintChecker

While we implemented taint propagation rules for several
builtin/standard functions, there's a natural desire for users to add
such rules to custom functions.

A series of patches will implement an option that allows users to
annotate their functions with taint propagation rules through a YAML
file. This one adds parsing of the configuration file, which may be
specified in the commands line with the analyzer config:
alpha.security.taint.TaintPropagation:Config. The configuration may
contain propagation rules, filter functions (remove taint) and sink
functions (give a warning if it gets a tainted value).

I also added a new header for future checkers to conveniently read YAML
files as checker options.

Differential Revision: https://reviews.llvm.org/D59555

llvm-svn: 367190