Add tests for llvm-disasm from the LLVM project

[langston-barrett]

See the included README for motivation.

This test gives a very minimal example of a file for which llvm-disasm does the wrong thing as compared to any version of llvm-dis:

$ nix-shell --pure -p llvm_38 --run "./dist/build/disasm-test/disasm-test ./disasm-test/llvm-tests/mdnodes-distinct-nodes-first.ll"                          

--- /run/user/1000/mdnodes-distinct-nodes-first2986-1.ll        2018-11-26 14:39:30.834876979 -0800                                                          
+++ /run/user/1000/mdnodes-distinct-nodes-first2986-2.ll        2018-11-26 14:39:30.836876956 -0800                                                          
@@ -1,3 +1,3 @@
-!named = !{!0
-!0 = !{!1
-!1 = distinct !{!0
+!named = !{!1
+!0 = distinct !{!1
+!1 = !{!0
./disasm-test/llvm-tests/mdnodes-distinct-nodes-first.ll:success