IR: Implement uselistorder assembly directives

Implement `uselistorder` and `uselistorder_bb` assembly directives,
which allow the use-list order to be recovered when round-tripping to
assembly.

This is the bulk of PR20515.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216025 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AsmParser/LLToken.h

@@ -180,6 +180,9 @@ namespace lltok {
     kw_extractelement, kw_insertelement, kw_shufflevector,
     kw_extractvalue, kw_insertvalue, kw_blockaddress,
 
+    // Use-list order directives.
+    kw_uselistorder, kw_uselistorder_bb,
+
     // Unsigned Valued tokens (UIntVal).
     GlobalID,          // @42
     LocalVarID,        // %42