LTO: Add API to choose whether to embed uselists

Reverse libLTO's default behaviour for preserving use-list order in
bitcode, and add API for controlling it.  The default setting is now
`false` (don't preserve them), which is consistent with `clang`'s
default behaviour.

Users of libLTO should call `lto_codegen_should_embed_uselists(CG,true)`
prior to calling `lto_codegen_write_merged_modules()` whenever the
output file isn't part of the production workflow in order to reproduce
results with subsequent calls to `llc`.

(I haven't added tests since `llvm-lto` (the test tool for LTO) doesn't
support bitcode output, and even if it did: there isn't actually a good
way to test whether a tool has passed the flag.  If the order is already
"natural" (if the order will already round-trip) then no use-list
directives are emitted at all.  At some point I'll circle back to add
tests to `llvm-as` (etc.) that they actually respect the flag, at which
point I can somehow add a test here as well.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235943 91177308-0d34-0410-b5e6-96231b3b80d8

lib/LTO/LTOCodeGenerator.cpp

@@ -202,8 +202,7 @@ bool LTOCodeGenerator::writeMergedModules(const char *path,
   }
 
   // write bitcode to it
-  WriteBitcodeToFile(IRLinker.getModule(), Out.os(),
-                     /* ShouldPreserveUseListOrder */ true);
+  WriteBitcodeToFile(IRLinker.getModule(), Out.os(), ShouldEmbedUselists);
   Out.os().close();
 
   if (Out.os().has_error()) {