this is (unfortunately) several changes mixed together:

1. Spell SectionFlags::Writeable as "Writable".
2. Add predicates for deriving SectionFlags from SectionKinds.
3. Sink ELF-specific getSectionPrefixForUniqueGlobal impl into
   ELFTargetAsmInfo.
4. Fix SectionFlagsForGlobal to know that BSS/ThreadBSS has the
   BSS bit set (the real fix for PR4619).
5. Fix isSuitableForBSS to not put globals with explicit sections
   set in BSS (which was the reason #4 wasn't fixed earlier).
6. Remove my previous hack for PR4619.

llvm-svn: 77085