DebugInfo: Move SectionLabel tracking into CU's addRange

This makes the SectionLabel handling more resilient - specifically for
future PROPELLER work which will have more CU ranges (rather than just
one per function).

Ultimately it might be nice to make this more general/resilient to
arbitrary labels (rather than relying on the labels being created for CU
ranges & then being reused by ranges, loclists, and possibly other
addresses). It's possible that other (non-rnglist/loclist) uses of
addresses will need the addresses to be in SectionLabels earlier (eg:
move the CU.addRange to be done on function begin, rather than function
end, so during function emission they are already populated for other
use).
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
index 69a39d5..c4e86df 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
@@ -328,6 +328,8 @@
 }
 
 void DwarfCompileUnit::addRange(RangeSpan Range) {
+  DD->insertSectionLabel(Range.Begin);
+
   bool SameAsPrevCU = this == DD->getPrevCU();
   DD->setPrevCU(this);
   // If we have no current ranges just add the range and return, otherwise,