[Remarks] Emit variable info in auto-init remarks
This enhances the auto-init remark with information about the variable
that is auto-initialized.
This is based of debug info if available, or alloca names (mostly for
auto-init.c:4:7: remark: Call to memset inserted by -ftrivial-auto-var-init. Memory operation size: 4096 bytes.Variables: var (4096 bytes). [-Rpass-missed=annotation-remarks] int var; ^
This allows to see things like partial initialization of a variable that
the optimizer won't be able to completely remove.
Differential Revision: https://reviews.llvm.org/D97734