Reland r319090, "COFF: Do not create SectionChunks for discarded comdat...

Reland r319090, "COFF: Do not create SectionChunks for discarded comdat sections." with a fix for debug sections.

If /debug was not specified, readSection will return a null
pointer for debug sections. If the debug section is associative with
another section, we need to make sure that the section returned from
readSection is not a null pointer before adding it as an associative
section.

Differential Revision: https://reviews.llvm.org/D40533

llvm-svn: 319133