MCFragment: Remove clearContents and uses of non-streaming doneAppending

Make the fixed-size part of MCFragment append-only to support allocating
content as trailing data. The `doneAppending` API is reserved by
MCStreamer API before finish and should not be used by the addrsig and
call-graph-profile features.

llvm/include/llvm/MC/MCSection.h

diff --git a/llvm/include/llvm/MC/MCSection.h b/llvm/include/llvm/MC/MCSection.h
index 953e6f7..87a8349 100644
--- a/llvm/include/llvm/MC/MCSection.h
+++ b/llvm/include/llvm/MC/MCSection.h
@@ -343,9 +343,9 @@
   bool getAllowAutoPadding() const { return AllowAutoPadding; }
   void setAllowAutoPadding(bool V) { AllowAutoPadding = V; }
 
-  // Content-related functions manage parent's storage using ContentStart and
+  //== Content-related functions manage parent's storage using ContentStart and
   // ContentSize.
-  void clearContents() { ContentEnd = ContentStart; }
+
   // Get a SmallVector reference. The caller should call doneAppending to update
   // `ContentEnd`.
   SmallVectorImpl<char> &getContentsForAppending() {

llvm/lib/MC/MachObjectWriter.cpp

diff --git a/llvm/lib/MC/MachObjectWriter.cpp b/llvm/lib/MC/MachObjectWriter.cpp
index 3298eef..48d2fc6 100644
--- a/llvm/lib/MC/MachObjectWriter.cpp
+++ b/llvm/lib/MC/MachObjectWriter.cpp
@@ -797,11 +797,8 @@
                      UndefinedSymbolData);
 
   if (!CGProfile.empty()) {
-    MCSection *CGProfileSection = getContext().getMachOSection(
-        "__LLVM", "__cg_profile", 0, SectionKind::getMetadata());
-    auto &Frag = *CGProfileSection->begin();
-    Frag.clearContents();
-    raw_svector_ostream OS(Frag.getContentsForAppending());
+    SmallString<0> Content;
+    raw_svector_ostream OS(Content);
     for (const MCObjectWriter::CGProfileEntry &CGPE : CGProfile) {
       uint32_t FromIndex = CGPE.From->getSymbol().getIndex();
       uint32_t ToIndex = CGPE.To->getSymbol().getIndex();
@@ -809,7 +806,9 @@
       support::endian::write(OS, ToIndex, W.Endian);
       support::endian::write(OS, CGPE.Count, W.Endian);
     }
-    Frag.doneAppending();
+    MCSection *Sec = getContext().getMachOSection("__LLVM", "__cg_profile", 0,
+                                                  SectionKind::getMetadata());
+    llvm::copy(OS.str(), Sec->curFragList()->Head->getContents().data());
   }
 
   unsigned NumSections = Asm.end() - Asm.begin();

llvm/lib/MC/WinCOFFObjectWriter.cpp

diff --git a/llvm/lib/MC/WinCOFFObjectWriter.cpp b/llvm/lib/MC/WinCOFFObjectWriter.cpp
index c69b8d6..6ad4334 100644
--- a/llvm/lib/MC/WinCOFFObjectWriter.cpp
+++ b/llvm/lib/MC/WinCOFFObjectWriter.cpp
@@ -1070,10 +1070,8 @@
 
   // Create the contents of the .llvm_addrsig section.
   if (Mode != DwoOnly && OWriter.getEmitAddrsigSection()) {
-    auto *Sec = getContext().getCOFFSection(".llvm_addrsig",
-                                            COFF::IMAGE_SCN_LNK_REMOVE);
-    auto *Frag = Sec->curFragList()->Head;
-    raw_svector_ostream OS(Frag->getContentsForAppending());
+    SmallString<0> Content;
+    raw_svector_ostream OS(Content);
     for (const MCSymbol *S : OWriter.AddrsigSyms) {
       if (!S->isRegistered())
         continue;
@@ -1088,15 +1086,15 @@
              "executePostLayoutBinding!");
       encodeULEB128(SectionMap[TargetSection]->Symbol->getIndex(), OS);
     }
-    Frag->doneAppending();
+    auto *Sec = getContext().getCOFFSection(".llvm_addrsig",
+                                            COFF::IMAGE_SCN_LNK_REMOVE);
+    Sec->curFragList()->Tail->setVarContents(OS.str());
   }
 
   // Create the contents of the .llvm.call-graph-profile section.
   if (Mode != DwoOnly && !OWriter.getCGProfile().empty()) {
-    auto *Sec = getContext().getCOFFSection(".llvm.call-graph-profile",
-                                            COFF::IMAGE_SCN_LNK_REMOVE);
-    auto *Frag = Sec->curFragList()->Head;
-    raw_svector_ostream OS(Frag->getContentsForAppending());
+    SmallString<0> Content;
+    raw_svector_ostream OS(Content);
     for (const auto &CGPE : OWriter.getCGProfile()) {
       uint32_t FromIndex = CGPE.From->getSymbol().getIndex();
       uint32_t ToIndex = CGPE.To->getSymbol().getIndex();
@@ -1104,7 +1102,9 @@
       support::endian::write(OS, ToIndex, W.Endian);
       support::endian::write(OS, CGPE.Count, W.Endian);
     }
-    Frag->doneAppending();
+    auto *Sec = getContext().getCOFFSection(".llvm.call-graph-profile",
+                                            COFF::IMAGE_SCN_LNK_REMOVE);
+    Sec->curFragList()->Tail->setVarContents(OS.str());
   }
 
   assignFileOffsets();