Reapply "DebugInfo: Don't put fission type units in comdat sections."

This recommits r208930, r208933, and r208975 (by reverting r209338) and
reverts r209529 (the FIXME to readd this functionality once the tools
were fixed) now that DWP has been fixed to cope with a single section
for all fission type units.

Original commit message:

"Since type units in the dwo file are handled by a debug aware tool,
they don't need to leverage the ELF comdat grouping to implement
deduplication. Avoid creating all the .group sections for these as a
space optimization."

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213956 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfUnit.h

@@ -579,6 +579,10 @@ public:
            sizeof(uint32_t);                               // Type DIE Offset
   }
   void initSection(const MCSection *Section);
+  // Bring in the base function (taking two args, including the section symbol)
+  // for use when building DWO type units (they don't go in unique comdat
+  // sections)
+  using DwarfUnit::initSection;
   DwarfCompileUnit &getCU() override { return CU; }
 
 protected: