Resubmit "[pdb] Allow zero-copy read support for symbol streams.""

Due to differences in template instantiation rules, it is not
portable to static_assert(false) inside of an invalid specialization
of a template.  Instead I just =delete the method so that it can't
be used, and leave a comment that it must be explicitly specialized.

llvm-svn: 271027
diff --git a/llvm/lib/DebugInfo/PDB/Raw/ModInfo.cpp b/llvm/lib/DebugInfo/PDB/Raw/ModInfo.cpp
index 9ccb7ed..67dc81d 100644
--- a/llvm/lib/DebugInfo/PDB/Raw/ModInfo.cpp
+++ b/llvm/lib/DebugInfo/PDB/Raw/ModInfo.cpp
@@ -67,6 +67,8 @@
                              // Null terminated Obj File Name
 };
 
+ModInfo::ModInfo() : Layout(nullptr) {}
+
 ModInfo::ModInfo(codeview::StreamRef Stream) : Layout(nullptr) {
   codeview::StreamReader Reader(Stream);
   if (auto EC = Reader.readObject(Layout)) {