[PATCH] LinkerScript: Add parsing of the EXTERN command

[Meador Inge]

In http://reviews.llvm.org/D8271#139387, @davide wrote:

> In order to not have this gap growing I propose that at least for new directives we implement both
>  semantic and parsing action at the same time, even in two different patches to facilitate review.


I did exactly that for this series.  See http://reviews.llvm.org/D8272.

I did *not* do it with the `MEMORY` command because I believe that will take quite a bit of infrastructure to implement, but I plan to do it.  For cases like `MEMORY` (where the eval piece is much more complicated) I think it is better to get the parsing bit out.  For cases like `EXTERN` I think both sides should be submitted at the same time.  I plan to always split the parse and eval portions into separate patches.


http://reviews.llvm.org/D8271

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