[MS][ARM64] Hoist __ImageBase handling into TargetLoweringObjectFileCOFF

All COFF targets should use @IMGREL32 relocations for symbol differences
against __ImageBase. Do the same for getSectionForConstant, so that
immediates lowered to globals get merged across TUs.

Patch by Chris January

Differential Revision: https://reviews.llvm.org/D47783

llvm-svn: 334523