[analyzer] Add annotation for functions taking user-facing strings

There was already a returns_localized_nsstring annotation to indicate
that the return value could be passed to UIKit methods that would
display them. However, those UIKit methods were hard-coded, and it was
not possible to indicate that other classes/methods in a code-base would
do the same.

The takes_localized_nsstring annotation can be put on function
parameters and selector parameters to indicate that those will also show
the string to the user.

Differential Revision: https://reviews.llvm.org/D35186

llvm-svn: 308012