uselistorder: -mllvm -preserve-bc-use-list-order => -emit-llvm-uselists

Stop relying on `cl::opt` to pass along the driver's decision to
preserve use-lists.  Create a new `-cc1` option called
`-emit-llvm-uselists` that does the right thing (when -emit-llvm-bc).
Note that despite its generic name, it *doesn't* do the right thing when
-emit-llvm (LLVM assembly) yet.  I'll hook that up soon.

This doesn't really change the behaviour of the driver.  The default is
still to preserve use-lists for `clang -emit-llvm` and `clang
-save-temps`, and nothing else.  But it stops relying on global state
(and also is a nicer interface for hackers using `clang -cc1`).

llvm-svn: 234962

clang/lib/Frontend/CompilerInvocation.cpp

@@ -405,6 +405,10 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
     // Default Dwarf version is 4 if we are generating debug information.
     Opts.DwarfVersion = 4;
 
+  if (const Arg *A =
+          Args.getLastArg(OPT_emit_llvm_uselists, OPT_no_emit_llvm_uselists))
+    Opts.EmitLLVMUseLists = A->getOption().getID() == OPT_emit_llvm_uselists;
+
   Opts.DisableLLVMOpts = Args.hasArg(OPT_disable_llvm_optzns);
   Opts.DisableRedZone = Args.hasArg(OPT_disable_red_zone);
   Opts.ForbidGuardVariables = Args.hasArg(OPT_fforbid_guard_variables);