[LTO API] add lto_codegen_set_module to set the destination module.

When debugging LTO issues with ld64, we use -save-temps to save the merged
optimized bitcode file, then invoke ld64 again on the single bitcode file to
speed up debugging code generation passes and ld64 stuff after code generation.

llvm linking a single bitcode file via lto_codegen_add_module will generate a
different bitcode file from the single input. With the newly-added
lto_codegen_set_module, we can make sure the destination module is the same as
the input.

lto_codegen_set_module will transfer the ownship of the module to code
generator.

rdar://19024554


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230290 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Linker/Linker.h

@@ -70,6 +70,9 @@ public:
   /// Returns true on error.
   bool linkInModule(Module *Src);
 
+  /// \brief Set the composite to the passed-in module.
+  void setModule(Module *Dst);
+
   static bool LinkModules(Module *Dest, Module *Src,
                           DiagnosticHandlerFunction DiagnosticHandler);