DebugInfo: Support for compressed debug info sections

1) When creating a .debug_* section and instead create a .zdebug_
   section.
2) When creating a fragment in a .zdebug_* section, make it a compressed
   fragment.
3) When computing the size of a compressed section, compress the data
   and use the size of the compressed data.
4) Emit the compressed bytes.

Also, check that only if a section has a compressed fragment, then that
is the only fragment in the section.

Assert-fail if the fragment's data is modified after it is compressed.

Initial review on llvm-commits by Eric Christopher and Rafael Espindola.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204958 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCObjectStreamer.cpp

@@ -20,6 +20,7 @@
 #include "llvm/MC/MCSection.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/MC/MCSectionELF.h"
 using namespace llvm;
 
 MCObjectStreamer::MCObjectStreamer(MCContext &Context, MCAsmBackend &TAB,
@@ -63,7 +64,11 @@ MCDataFragment *MCObjectStreamer::getOrCreateDataFragment() const {
   // When bundling is enabled, we don't want to add data to a fragment that
   // already has instructions (see MCELFStreamer::EmitInstToData for details)
   if (!F || (Assembler->isBundlingEnabled() && F->hasInstructions())) {
-    F = new MCDataFragment();
+    const auto *Sec = dyn_cast<MCSectionELF>(&getCurrentSectionData()->getSection());
+    if (Sec && Sec->getSectionName().startswith(".zdebug_"))
+      F = new MCCompressedFragment();
+    else
+      F = new MCDataFragment();
     insert(F);
   }
   return F;