COFF/ELF: Place llvm.global_ctors elements in llvm.used if comdat is used

On ELF, an SHT_INIT_ARRAY outside a section group is a GC root. The current
codegen abuses SHT_INIT_ARRAY in a section group to mean a GC root.

On PE/COFF, the dynamic initialization for `__declspec(selectany)` in a comdat
can be garbage collected by `-opt:ref`.

Call `addUsedGlobal` for the two cases to fix the abuse/bug.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D106925

(cherry picked from commit 828767f3)