Restore LLVMLinkModules C API until it is properly deprecated.

Add the enum "LLVMLinkerMode" back for backwards-compatibility and add the
linker mode parameter back to the "LLVMLinkModules" function. The paramter is
ignored and has no effect.

Patch provided by: Filip Pizlo
Reviewed by: Rafael and Sean

llvm-svn: 230988
diff --git a/llvm/include/llvm-c/Linker.h b/llvm/include/llvm-c/Linker.h
index cedde5e..9f98a33 100644
--- a/llvm/include/llvm-c/Linker.h
+++ b/llvm/include/llvm-c/Linker.h
@@ -20,13 +20,23 @@
 extern "C" {
 #endif
 
+/* This enum is provided for backwards-compatibility only. It has no effect. */
+typedef enum {
+  LLVMLinkerDestroySource = 0, /* This is the default behavior. */
+  LLVMLinkerPreserveSource_Removed = 1 /* This option has been deprecated and
+                                          should not be used. */
+} LLVMLinkerMode;
+
 /* Links the source module into the destination module, taking ownership
  * of the source module away from the caller. Optionally returns a
  * human-readable description of any errors that occurred in linking.
  * OutMessage must be disposed with LLVMDisposeMessage. The return value
- * is true if an error occurred, false otherwise. */
+ * is true if an error occurred, false otherwise.
+ *
+ * Note that the linker mode parameter \p Unused is no longer used, and has
+ * no effect. */
 LLVMBool LLVMLinkModules(LLVMModuleRef Dest, LLVMModuleRef Src,
-                         unsigned Unused, char **OutMessage);
+                         LLVMLinkerMode Unused, char **OutMessage);
 
 #ifdef __cplusplus
 }