[PATCH] [InputGraph][Gnu] Add Linkerscript support.

[Shankar Kalpathi Easwaran]

  I completely agree that this patch is combining other functionalities. But all these patches as a whole need to be used inorder to run the tests. This was the only reason of merging them all-together in one patch.

  I will split this patch into 4 individual commits before I commit.

  a) InputGraph functionality to expand elements
  b) InputGraph functionality to set the hidden attribute
  c) Add an API to linkingcontext on how undefined atoms from shared libraries need to be processed
  d) Add LinkerScript nodes.

  I dont know how to add this in the testing environment as GnuLD driver would try to look at the files before it adds to the InputGraph.

  Any inputs on this would be really helpful.

http://llvm-reviews.chandlerc.com/D2225