IR: Add COMDATs to the IR

This new IR facility allows us to represent the object-file semantic of
a COMDAT group.

COMDATs allow us to tie together sections and make the inclusion of one
dependent on another. This is required to implement features like MS
ABI VFTables and optimizing away certain kinds of initialization in C++.

This functionality is only representable in COFF and ELF, Mach-O has no
similar mechanism.

Differential Revision: http://reviews.llvm.org/D4178

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211920 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/LLVMBitCodes.h

@@ -71,7 +71,8 @@ namespace bitc {
     // MODULE_CODE_PURGEVALS: [numvals]
     MODULE_CODE_PURGEVALS   = 10,
 
-    MODULE_CODE_GCNAME      = 11   // GCNAME: [strchr x N]
+    MODULE_CODE_GCNAME      = 11,  // GCNAME: [strchr x N]
+    MODULE_CODE_COMDAT      = 12,  // COMDAT: [selection_kind, name]
   };
 
   /// PARAMATTR blocks have code for defining a parameter attribute set.
@@ -376,6 +377,14 @@ namespace bitc {
     ATTR_KIND_JUMP_TABLE = 40
   };
 
+  enum ComdatSelectionKindCodes {
+    COMDAT_SELECTION_KIND_ANY = 1,
+    COMDAT_SELECTION_KIND_EXACT_MATCH = 2,
+    COMDAT_SELECTION_KIND_LARGEST = 3,
+    COMDAT_SELECTION_KIND_NO_DUPLICATES = 4,
+    COMDAT_SELECTION_KIND_SAME_SIZE = 5,
+  };
+
 } // End bitc namespace
 } // End llvm namespace