[COFF] Fix assembly output of comdat sections without an attached symbol

Since SVN r335286, the .xdata sections are produced without an attached
symbol, which requires using a different syntax when printing assembly
output.

Instead of the usual syntax of '.section <name>,"dr",discard,<symbol>',
use '.section <name>,"dr"' + '.linkonce discard' (which is what GCC
uses for all assembly output).

This fixes PR38254.

Differential Revision: https://reviews.llvm.org/D49651

llvm-svn: 337756