[WIP][Attributor] AAReachability Attribute

[WIP][Attributor] AAReachability Attribute

     Summary: Working towards Johannes's suggestion for fixme, in Attributor's Noalias attribute deduction.
(ii) Check whether the value is captured in the scope using AANoCapture.
FIXME: This is conservative though, it is better to look at CFG and
// check only uses possibly executed before this call site.

A Reachability abstract attribute answers the question "does execution at point A potentially reach point B". If this question is answered with false for all other uses of the value that might be captured, we know it is not *yet* captured and can continue with the noalias deduction. Currently, information AAReachability provides is completely pessimistic.

    Reviewers: jdoerfert

    Reviewed By: jdoerfert

    Subscribers: uenoku, sstefan1, hiraditya, llvm-commits

    Differential Revision: https://reviews.llvm.org/D70233