[PATCH] Fix Weak External symbol handling.

[Shankar Kalpathi Easwaran]

  The main problem I have here understanding is

  handleFile is called when looking at kindObject files. If I have an undefined symbol in the object file with a fallback atom, i think its too early to use the fallback atom instead of the undefined atom, when looking at that single object file.

  For example :

  Say you have

  lld -flavor link hello.o libc.a (the link line may not look like this, but only taking this as an example).

  Currently when the resolver reads in hello.o and say printf has a fallback to puts, its too early to replace undefined atom printf with puts, when you have just read hello.o.

  Does link.exe have this same behavior ?

http://llvm-reviews.chandlerc.com/D2162