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

include/llvm/LTO/LTOCodeGenerator.h

@@ -80,6 +80,7 @@ struct LTOCodeGenerator {
   void setOptLevel(unsigned optLevel) { OptLevel = optLevel; }
 
   void setShouldInternalize(bool Value) { ShouldInternalize = Value; }
+  void setShouldEmbedUselists(bool Value) { ShouldEmbedUselists = Value; }
 
   void addMustPreserveSymbol(const char *sym) { MustPreserveSymbols[sym] = 1; }
 
@@ -176,6 +177,7 @@ private:
   void *DiagContext = nullptr;
   LTOModule *OwnedModule = nullptr;
   bool ShouldInternalize = true;
+  bool ShouldEmbedUselists = false;
 };
 }
 #endif