[PATCH] Add preload method.

[Shankar Kalpathi Easwaran]

a) The undefined symbol could very well be in a shared library or a regular ELF object, which is already parsed by a thread. 
b) There might be more than one archive library or a .o that would have the same symbol name.
c) Weak symbol resolution has to be taken care as well, as the symbol name would be the same, and symbols may be picked up from a different archive on how the weak symbol was resolved.
d) The inputGraph is agnostic to what input in the graph contains the symbol name.

We could have a background thread, that could be designated to **only parse archive** files by looking at undefined symbols.

What do you think ?

================
Comment at: include/lld/Core/InputGraph.h:85
@@ +84,3 @@
+  /// file is requested from the core linker we can respond to it quickly.
+  /// It does not change the meaining of the code. In particualr this
+  /// function should not have any observable side effect.
----------------
spell errors.

http://reviews.llvm.org/D5340