Move alignment from MCSectionData to MCSection.

This starts merging MCSection and MCSectionData.

There are a few issues with the current split between MCSection and
MCSectionData.

* It optimizes the the not as important case. We want the production
of .o files to be really fast, but the split puts the information used
for .o emission in a separate data structure.

* The ELF/COFF/MachO hierarchy is not represented in MCSectionData,
leading to some ad-hoc ways to represent the various flags.

* It makes it harder to remember where each item is.

The attached patch starts merging the two by moving the alignment from
MCSectionData to MCSection.

Most of the patch is actually just dropping 'const', since
MCSectionData is mutable, but MCSection was not.

llvm-svn: 237936
diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
index 31e9320..9e0cc6b 100644
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -188,7 +188,7 @@
   SwitchSection(getContext().getObjectFileInfo()->getTextSection());
 }
 
-void MCStreamer::AssignSection(MCSymbol *Symbol, const MCSection *Section) {
+void MCStreamer::AssignSection(MCSymbol *Symbol, MCSection *Section) {
   if (Section)
     Symbol->setSection(*Section);
   else
@@ -640,9 +640,9 @@
 void MCStreamer::EmitELFSize(MCSymbol *Symbol, const MCExpr *Value) {}
 void MCStreamer::EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
                                        unsigned ByteAlignment) {}
-void MCStreamer::EmitTBSSSymbol(const MCSection *Section, MCSymbol *Symbol,
+void MCStreamer::EmitTBSSSymbol(MCSection *Section, MCSymbol *Symbol,
                                 uint64_t Size, unsigned ByteAlignment) {}
-void MCStreamer::ChangeSection(const MCSection *, const MCExpr *) {}
+void MCStreamer::ChangeSection(MCSection *, const MCExpr *) {}
 void MCStreamer::EmitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol) {}
 void MCStreamer::EmitBytes(StringRef Data) {}
 void MCStreamer::EmitValueImpl(const MCExpr *Value, unsigned Size,
@@ -664,8 +664,7 @@
 void MCStreamer::FinishImpl() {}
 void MCStreamer::EmitBundleUnlock() {}
 
-void MCStreamer::SwitchSection(const MCSection *Section,
-                               const MCExpr *Subsection) {
+void MCStreamer::SwitchSection(MCSection *Section, const MCExpr *Subsection) {
   assert(Section && "Cannot switch to a null section!");
   MCSectionSubPair curSection = SectionStack.back().first;
   SectionStack.back().second = curSection;
@@ -679,7 +678,7 @@
   }
 }
 
-MCSymbol *MCStreamer::endSection(const MCSection *Section) {
+MCSymbol *MCStreamer::endSection(MCSection *Section) {
   // TODO: keep track of the last subsection so that this symbol appears in the
   // correct place.
   MCSymbol *Sym = Section->getEndSymbol(Context);