[COFF] Handle comdat sections without leader symbols

Discard them unless they have been associated by other means (yet
uimplemented).

According to MS link.exe, such sections are illegal, but MinGW setups
use them in their take on associative comdats.

This avoids leaving references to the bogus SectionChunk* PendingComdat,
which cannot be dereferenced.

This fixes PR38183.

Differential Revision: https://reviews.llvm.org/D49653

llvm-svn: 338064