[COFF] Bounds check relocations

Summary:
This would have caught the invalid object file I used in my test case in
r307726. The OOB was only caught by ASan later, which is slow and
doesn't work on some platforms. LLD should do some basic input
validation itself. This check isn't perfect, so relocations can reach
OOB by up to seven bytes, but it's better than what we had and probably
cheap.

Reviewers: ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35371

llvm-svn: 307948