[DebugInfo/DWARF] [3/4] Rename DWARFUnitSection to DWARFUnitVector. NFC

This is patch 3 of 4 NFC refactorings to handle type units and compile
units more consistently and with less concern about the object-file
section that they came from.

Patch 3 simply renames DWARFUnitSection to DWARFUnitVector, as the
object-file section of a unit is nearly irrelevant now.

Differential Revision: https://reviews.llvm.org/D49743

llvm-svn: 338632
diff --git a/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp b/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
index 904ceab..654d205 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
@@ -350,7 +350,7 @@
 DWARFDie
 DWARFDie::getAttributeValueAsReferencedDie(dwarf::Attribute Attr) const {
   if (auto SpecRef = toReference(find(Attr))) {
-    if (auto SpecUnit = U->getUnitSection().getUnitForOffset(*SpecRef))
+    if (auto SpecUnit = U->getUnitVector().getUnitForOffset(*SpecRef))
       return SpecUnit->getDIEForOffset(*SpecRef);
   }
   return DWARFDie();