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-c/lto.h

@@ -40,7 +40,7 @@ typedef bool lto_bool_t;
  * @{
  */
 
-#define LTO_API_VERSION 14
+#define LTO_API_VERSION 15
 
 /**
  * \since prior to LTO_API_VERSION=3
@@ -558,6 +558,18 @@ extern void
 lto_codegen_set_should_internalize(lto_code_gen_t cg,
                                    lto_bool_t ShouldInternalize);
 
+/**
+ * \brief Set whether to embed uselists in bitcode.
+ *
+ * Sets whether \a lto_codegen_write_merged_modules() should embed uselists in
+ * output bitcode.  This should be turned on for all -save-temps output.
+ *
+ * \since LTO_API_VERSION=15
+ */
+extern void
+lto_codegen_set_should_embed_uselists(lto_code_gen_t cg,
+                                      lto_bool_t ShouldEmbedUselists);
+
 #ifdef __cplusplus
 }
 #endif