Add register lookup as another fallback computation for address-expressions (#85492)

The idea behind the address-expression is that it handles all the common
expressions that produce addresses. It

Add register lookup as another fallback computation for address-expressions (#85492)

The idea behind the address-expression is that it handles all the common
expressions that produce addresses. It handles actual valid expressions
that return a scalar, and it handles useful cases that the various
source languages don't support. At present, the fallback handles:

<symbol_name>{+-}<offset>

which isn't valid C but is very handy.

This patch adds handling of:

$<reg_name>

and

$<reg_name>{+-}<offset>

That's kind of pointless in C because the C expression parser handles
that expression already. But some languages don't have a straightforward
way to represent register values like this (swift) so having this
fallback is quite a quality of life improvement.

I added a test which tests that I didn't mess up either of these
fallbacks, though it doesn't test the actually handling of registers
that I added, since the expression parser for C succeeds in that case
and returns before this code gets run.

I will add a test on the swift fork for that checks that this works the
same way for a swift frame after this check.

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