Compute A-B if both A and B are in the same comdat section.

Compute A-B if both A and B are in the same comdat section.

Part of pr23272.

A small annoyance with the assembly syntax we implement is that given an
expression there is no way to know if what is desired is the value of that
expression for the symbols in this file or for the final values of those
symbols in a link.

The first case is useful for use in sections that get discarded or ignored
if the section they are describing is discarded.

For axample, consider A-B where A and B are in the same comdat section.

We can compute the value of the difference in the section that is present in
the current .o and if that section survives to the final DSO the value will
still will be correct.

But the section is in a comdat. Another section from another object file
might be used istead. We know that that section will define A and B, but
we have no idea what the value of A-B might be.

In practice we have to assume that the intention is to compute the value
in the current section since otherwise the is no way to create something like
the debug aranges section.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235222 91177308-0d34-0410-b5e6-96231b3b80d8