DebugInfo: Add metadata support for disabling DWARF pub sections

In cases where the debugger load time is a worthwhile tradeoff (or less
costly - such as loading from a DWP instead of a variety of DWOs
(possibly over a high-latency/distributed filesystem)) against object
file size, it can be reasonable to disable pubnames and corresponding
gdb-index creation in the linker.

A backend-flag version of this was implemented for NVPTX in
D44385/r327994 - which was fine for NVPTX which wouldn't mix-and-match
CUs. Now that it's going to be a user-facing option (likely powered by
"-gno-pubnames", the same as GCC) it should be encoded in the
DICompileUnit so it can vary per-CU.

After this, likely the NVPTX support should be migrated to the metadata
& the previous flag implementation should be removed.

Reviewers: aprantl

Differential Revision: https://reviews.llvm.org/D50213

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339939 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/MetadataLoader.cpp

@@ -1393,7 +1393,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
         Record.size() <= 14 ? 0 : Record[14],
         Record.size() <= 16 ? true : Record[16],
         Record.size() <= 17 ? false : Record[17],
-        Record.size() <= 18 ? false : Record[18]);
+        Record.size() <= 18 ? 0 : Record[18]);
 
     MetadataList.assignValue(CU, NextMetadataNo);
     NextMetadataNo++;