[PECOFF] Support linking against DLL.

Summary:
This patch adds a new pass, IdataPass, to transform shared atom references
to real references and to construct the .idata section data. With this patch
lld can produce a working Hello World program by linking it against
kernel32.dll and user32.dll.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1096
[PECOFF][Writer] Emit .reloc section.

Summary:
Emit .reloc section. This is the first step to support DLL creation. The
executable doesn't need .reloc section, but the DLL does.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1126