PECOFF: Fix base relocation for ImageBase.

This is yet another edge case of base relocation for symbols. Absolute
symbols are in general not target of base relocation because absolute
atom is a way to point to a specific memory location. In r229816, I
removed entries for absolute atoms from the base relocation table
(so that they won't be fixed by the loader).

However, there was one exception -- ImageBase. ImageBase points to the
start address of the current image in memory. That needs to be fixed up
at load time. This patch is to treat the symbol in a special manner.

llvm-svn: 229961