Tweak insecureAPI analyzer checks to have the ability to be individually disabled.

The solution is a bit inefficient: it creates N checkers, one for each check, and
each check does a dispatch on the function name.  This is redundant, but we can fix
this once we have the proper ability to enable/disable subchecks.

Fixes <rdar://problem/11780180>.

llvm-svn: 159459