Reland: [Remarks] Add an LLVM-bitstream-based remark serializer

Add a new serializer, using a binary format based on the LLVM bitstream
format.

This format provides a way to serialize the remarks in two modes:

1) Separate mode: the metadata is separate from the remark entries.
2) Standalone mode: the metadata and the remark entries are in the same
file.

The format contains:

* a meta block: container version, container type, string table,
external file path, remark version
* a remark block: type, remark name, pass name, function name, debug
file, debug line, debug column, hotness, arguments (key, value, debug
file, debug line, debug column)

A string table is required for this format, which will be dumped in the
meta block to be consumed before parsing the remark blocks.

On clang itself, we noticed a size reduction of 13.4x compared to YAML,
and a compile-time reduction of between 1.7% and 3.5% on CTMark.

Differential Revision: https://reviews.llvm.org/D63466

Original llvm-svn: 367364
Revert llvm-svn: 367370

llvm-svn: 367372
diff --git a/llvm/lib/Remarks/RemarkSerializer.cpp b/llvm/lib/Remarks/RemarkSerializer.cpp
index 73cec4f..0d5acbe 100644
--- a/llvm/lib/Remarks/RemarkSerializer.cpp
+++ b/llvm/lib/Remarks/RemarkSerializer.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Remarks/RemarkSerializer.h"
+#include "llvm/Remarks/BitstreamRemarkSerializer.h"
 #include "llvm/Remarks/YAMLRemarkSerializer.h"
 
 using namespace llvm;
@@ -27,6 +28,8 @@
     return llvm::make_unique<YAMLRemarkSerializer>(OS, Mode);
   case Format::YAMLStrTab:
     return llvm::make_unique<YAMLStrTabRemarkSerializer>(OS, Mode);
+  case Format::Bitstream:
+    return llvm::make_unique<BitstreamRemarkSerializer>(OS, Mode);
   }
   llvm_unreachable("Unknown remarks::Format enum");
 }
@@ -45,6 +48,9 @@
   case Format::YAMLStrTab:
     return llvm::make_unique<YAMLStrTabRemarkSerializer>(OS, Mode,
                                                          std::move(StrTab));
+  case Format::Bitstream:
+    return llvm::make_unique<BitstreamRemarkSerializer>(OS, Mode,
+                                                        std::move(StrTab));
   }
   llvm_unreachable("Unknown remarks::Format enum");
 }