at the point when they become proper globals.

We need to allow BitCasts between pointer types to different prim types,
but that means we need to catch the problem at a later stage, i.e. when
loading the values.

Fixes https://github.com/llvm/llvm-project/issues/158527
Fixes https://github.com/llvm/llvm-project/issues/163778

Dummy variables have an entry in `Program::Globals`, but they are not
added to `GlobalIndices`. When registering redeclarations, we used to
only patch up the global indices, but that left the dummy variables
alone. Update the dummy variables of all redeclarations as well.


Fixes https://github.com/llvm/llvm-project/issues/165952

One iteration of this loop might've already fixed up the pointers of
coming globals, so check for that explicitly.

Fixes https://github.com/llvm/llvm-project/issues/164151

which moves all the block's pointers to a new block.

And make use of those.

These changes are split from prior PR #155028, in order to decrease the
size of that PR and facilitate review.

We just initialized the entire string, so use this function instead.

This changes a bunch of places which use getAs<TagType>, including
derived types, just to obtain the tag definition.

This is preparation for #155028, offloading all the changes that PR used
to introduce which don't depend on any new helpers.

So we can save ourselves writing PointeeStorage all the time.