Bitcode: Serialize (and recover) use-list order

Predict and serialize use-list order in bitcode.  This makes the option
`-preserve-bc-use-list-order` work *most* of the time, but this is still
experimental.

  - Builds a full value-table up front in the writer, sets up a list of
    use-list orders to write out, and discards the table.  This is a
    simpler first step than determining the order from the various
    overlapping IDs of values on-the-fly.

  - The shuffles stored in the use-list order list have an unnecessarily
    large memory footprint.

  - `blockaddress` expressions cause functions to be materialized
    out-of-order.  For now I've ignored this problem, so use-list orders
    will be wrong for constants used by functions that have block
    addresses taken.  There are a couple of ways to fix this, but I
    don't have a concrete plan yet.

  - When materializing functions lazily, the use-lists for constants
    will not be correct.  This use case is out of scope: what should the
    use-list order be, if it's incomplete?

This is part of PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214125 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/LLVMBitCodes.h

@@ -330,7 +330,8 @@ namespace bitc {
   };
 
   enum UseListCodes {
-    USELIST_CODE_ENTRY = 1   // USELIST_CODE_ENTRY: TBD.
+    USELIST_CODE_DEFAULT = 1, // DEFAULT: [index..., value-id]
+    USELIST_CODE_BB      = 2  // BB: [index..., bb-id]
   };
 
   enum AttributeKindCodes {