Begin adding experimental support for preserving use-list ordering of bitcode

files.  First, add a new block USELIST_BLOCK to the bitcode format.  This is 
where USELIST_CODE_ENTRYs will be stored.  The format of the USELIST_CODE_ENTRYs
have not yet been defined.  Add support in the BitcodeReader for parsing the
USELIST_BLOCK.
Part of rdar://9860654 and PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146078 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/LLVMBitCodes.h

@@ -41,7 +41,9 @@ namespace bitc {
     METADATA_BLOCK_ID,
     METADATA_ATTACHMENT_ID,
     
-    TYPE_BLOCK_ID_NEW
+    TYPE_BLOCK_ID_NEW,
+
+    USELIST_BLOCK_ID
   };
 
 
@@ -311,6 +313,10 @@ namespace bitc {
     FUNC_CODE_INST_STOREATOMIC = 42  // STORE: [ptrty,ptr,val, align, vol
                                      //         ordering, synchscope]
   };
+
+  enum UseListCodes {
+    USELIST_CODE_ENTRY = 1   // USELIST_CODE_ENTRY: TBD.
+  };
 } // End bitc namespace
 } // End llvm namespace