[IRGen] Add !annotation metadata for auto-init stores.

This patch updates Clang's IRGen to add !annotation nodes with an
"auto-init" annotation to all stores for auto-initialization.

As discussed in 'RFC: Combining Annotation Metadata and Remarks'
(http://lists.llvm.org/pipermail/llvm-dev/2020-November/146393.html)
this allows using optimization remarks to track down where auto-init
code was inserted (and not removed by optimizations).

There are a few cases in the tests where !annotation gets dropped by
optimizations. Those optimizations will be updated in subsequent
patches.

This patch is based on a patch by Francis Visoiu Mistrih.

Reviewed By: thegameg, paquette

Differential Revision: https://reviews.llvm.org/D91417