[ASTMatchers] Add forCallable(), a generalization of forFunction().

The new matcher additionally covers blocks and Objective-C methods.

This matcher actually makes sure that the statement truly belongs
to that declaration's body. forFunction() incorrectly reported that
a statement in a nested block belonged to the surrounding function.

forFunction() is now deprecated due to the above footgun, in favor of
forCallable(functionDecl()) when only functions need to be considered.

Differential Revision: https://reviews.llvm.org/D102213