[lldb][NFC] New names for the two RegisterLocation classes (#109611)

[jasonmolenda]

lldb has two RegisterLocation classes that do slightly different things.

UnwindPlan::Row::RegisterLocation (new: AbstractRegisterLocation) has a
description of how to find a register's value or location, not specific
to a particular stopping point. It may say that at a given offset into a
function, the caller's register has been spilled to stack memory at CFA
minus an offset. Or it may say that the caller's register is at a DWARF
exprssion.

UnwindLLDB::RegisterLocation (new: ConcreteRegisterLocation) is a
specific address where the register is currently stored, or the register
it has been copied into, or its value at this point in the current
function execution.

When lldb stops in a function, it interprets the
AbstractRegisterLocation's instructions using the register context and
stack memory, to create the ConcreteRegisterLocation at this point in
time for this stack frame.

I'm not thrilled with AbstractRegisterLocation and
ConcreteRegisterLocation, but it's better than the same name and it will
be easier to update them if someone suggests a better pair.

(cherry picked from commit 3336d73)

[jasonmolenda]

@felipepiovezan do you want this on rebranch.

[felipepiovezan]

@felipepiovezan do you want this on rebranch.

Yes please! I am actively using those classes there, so it would reduce merge conflicts! :)