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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230988 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Linker/LinkModules.cpp

@@ -1777,7 +1777,7 @@ bool Linker::LinkModules(Module *Dest, Module *Src) {
 //===----------------------------------------------------------------------===//
 
 LLVMBool LLVMLinkModules(LLVMModuleRef Dest, LLVMModuleRef Src,
-                         unsigned Unused, char **OutMessages) {
+                         LLVMLinkerMode Unused, char **OutMessages) {
   Module *D = unwrap(Dest);
   std::string Message;
   raw_string_ostream Stream(Message);