[PATCH] Add default parameter to MCSymbol accessors to control IsUsed

[Rafael Espíndola via llvm-commits]

> Could someone clarify what exactly "invalid reassignment to non-absolute variable" means? It's starting to look like this error should never pop up.

I think that dates from the early days of MC. My understanding is that
it was added just as a safety so that we have to reason about code
like

foo = bar
.long foo
foo = zed
.long foo

Should that produce two relocations with bar, two with foo or one each?

Cheers,
Rafael