Fix a major regression on COFF targets introduced by r103267: 'discardable'...

Fix a major regression on COFF targets introduced by r103267: 'discardable' section means that it is used only during the program load and can be discarded afterwards.
This way *only* debug sections can be discarded, but not the opposite. Seems like the copy-and-pasto from ELF code, since there it contains the reverse flag ('alloc').

llvm-svn: 107658