[analyzer] Explicitly register NoStoreFuncVisitor from alpha.unix.cst… (#108373)

…ring.UninitRead

This is a drastic simplification of #106982. If you read that patch,
this is the same thing with all BugReporterVisitors.cpp and
SValBuilder.cpp changes removed! (since all replies came regarding
changed to those files, I felt the new PR was justified)

The patch was inspired by a pretty poor bug report on FFMpeg:

![image](https://github.com/user-attachments/assets/8f4e03d8-45a4-4ea2-a63d-3ab78d097be9)

In this bug report, block is uninitialized, hence the bug report that it
should not have been passed to memcpy. The confusing part is in line 93,
where block was passed as a non-const pointer to seq_unpack_rle_block,
which was obviously meant to initialize block. As developers, we know
that clang likely didn't skip this function and found a path of
execution on which this initialization failed, but NoStoreFuncVisitor
failed to attach the usual "returning without writing to block" message.

I fixed this by instead of tracking the entire array, I tracked the
actual element which was found to be uninitialized (Remember, we
heuristically only check if the first and last-to-access element is
initialized, not the entire array). This is how the bug report looks
now, with 'seq_unpack_rle_block' having notes describing the path of
execution and lack of a value change:

![image](https://github.com/user-attachments/assets/8de5d101-052e-4ecb-9cd9-7c29724333d2)

![image](https://github.com/user-attachments/assets/8bf52a95-62de-44e7-aef8-03a46a3fa08e)

Since NoStoreFuncVisitor was a TU-local class, I moved it back to
BugReporterVisitors.h, and registered it manually in CStringChecker.cpp.
This was done because we don't have a good trackRegionValue() function,
only a trackExpressionValue() function. We have an expression for the
array, but not for its first (or last-to-access) element, so I only had
a MemRegion on hand.

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