Sink PrevCU updating into DwarfUnit::addRange to ensure consistency

So that the two operations in DwarfDebug couldn't get separated (because
I accidentally separated them in some work in progress), put them
together. While we're here, move DwarfUnit::addRange to
DwarfCompileUnit, since it's not relevant to type units.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217468 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -1709,7 +1709,6 @@ void DwarfDebug::endFunction(const MachineFunction *MF) {
   // Add the range of this function to the list of ranges for the CU.
   RangeSpan Span(FunctionBeginSym, FunctionEndSym);
   TheCU.addRange(std::move(Span));
-  PrevCU = &TheCU;
 
   // Clear debug info
   // Ownership of DbgVariables is a bit subtle - ScopeVariables owns all the