[llvm-objdump] Print "..." instead of random data for virtual sections

When disassembling with -D, skip virtual sections by printing "..." for
each symbol.

This patch also implements `MachOObjectFile::isSectionVirtual`.

Test case comes from:

```
.zerofill __DATA,__common,_data64unsigned,472,3
```

Differential Revision: https://reviews.llvm.org/D45824

llvm-svn: 330342

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/tools/llvm-objdump/llvm-objdump.cpp b/llvm/tools/llvm-objdump/llvm-objdump.cpp
index 718d5749412..5bea3a7860b 100644
--- a/llvm/tools/llvm-objdump/llvm-objdump.cpp
+++ b/llvm/tools/llvm-objdump/llvm-objdump.cpp
@@ -1494,6 +1494,13 @@ static void DisassembleObject(const ObjectFile *Obj, bool InlineRelocs) {
 
       outs() << '\n' << std::get<1>(Symbols[si]) << ":\n";
 
+      // Don't print raw contents of a virtual section. A virtual section
+      // doesn't have any contents in the file.
+      if (Section.isVirtual()) {
+        outs() << "...\n";
+        continue;
+      }
+
 #ifndef NDEBUG
       raw_ostream &DebugOut = DebugFlag ? dbgs() : nulls();
 #else