Store a bit in MCSection saying if it was registered with MCAssembler.

With this we can replace a SetVector with a plain std::vector.

llvm-svn: 238706
diff --git a/llvm/lib/MC/MCSection.cpp b/llvm/lib/MC/MCSection.cpp
index d39cb5d..9152f2b 100644
--- a/llvm/lib/MC/MCSection.cpp
+++ b/llvm/lib/MC/MCSection.cpp
@@ -21,7 +21,7 @@
 
 MCSection::MCSection(SectionVariant V, SectionKind K, MCSymbol *Begin)
     : Begin(Begin), BundleGroupBeforeFirstInst(false), HasInstructions(false),
-      Variant(V), Kind(K) {}
+      IsRegistered(false), Variant(V), Kind(K) {}
 
 MCSymbol *MCSection::getEndSymbol(MCContext &Ctx) {
   if (!End)