[COFF] Don't produce base relocs for discardable sections

Dwarf debug info contains some data that contains absolute addresses.
Since these sections are discardable and aren't loaded at runtime,
there's no point in adding base relocations for them.

This makes sure that after stripping out dwarf debug info, there are no
base relocations that point to nonexistent sections.

Differential Revision: https://reviews.llvm.org/D49350

llvm-svn: 337438