[NvlinkWrapper] Add relocatable link support to LTO pass

Summary:
We need `-r` to work so we can preserve symbols.

Author: jhuber6

clang/tools/clang-nvlink-wrapper/ClangNVLinkWrapper.cpp

@@ -598,10 +598,11 @@ Expected<SmallVector<StringRef>> getInput(const ArgList &Args) {
         Res.Prevailing = !Sym.isUndefined() && ObjSym.File == *BitcodeFile;
 
         // We need LTO to preseve the following global symbols:
-        // 1) Symbols used in regular objects.
-        // 2) Prevailing symbols that are needed visible to the gpu runtime.
+        // 1) All symbols during a relocatable link.
+        // 2) Symbols used in regular objects.
+        // 3) Prevailing symbols that are needed visible to the gpu runtime.
         Res.VisibleToRegularObj =
-            ObjSym.UsedInRegularObj ||
+            Args.hasArg(OPT_relocatable) || ObjSym.UsedInRegularObj ||
             (Res.Prevailing &&
              (Sym.getVisibility() != GlobalValue::HiddenVisibility &&
               !Sym.canBeOmittedFromSymbolTable()));

clang/tools/clang-nvlink-wrapper/NVLinkOpts.td

@@ -73,6 +73,10 @@ def plugin_opt : Joined<["--", "-"], "plugin-opt=">, Flags<[WrapperOnlyOption]>,
 def save_temps : Flag<["--", "-"], "save-temps">,
   Flags<[WrapperOnlyOption]>, HelpText<"Save intermediate results">;
 
+def relocatable : Flag<["--", "-"], "relocatable">,
+  Flags<[WrapperOnlyOption]>, HelpText<"Perform a relocatable link (LTO only)">;
+def r : Flag<["-"], "r">, Flags<[WrapperOnlyOption]>, Alias<relocatable>;
+
 def whole_archive : Flag<["--", "-"], "whole-archive">,
   Flags<[WrapperOnlyOption, HelpHidden]>;
 def no_whole_archive : Flag<["--", "-"], "no-whole-archive">,
@@ -83,8 +87,7 @@ def mllvm : Separate<["-"], "mllvm">, Flags<[WrapperOnlyOption]>,
   HelpText<"Arguments passed to LLVM, including Clang invocations, for which "
            "the '-mllvm' prefix is preserved. Use '-mllvm --help' for a list "
            "of options.">;
-def mllvm_EQ : Joined<["-"], "mllvm=">, Flags<[HelpHidden]>,
-  Alias<mllvm>;
+def mllvm_EQ : Joined<["-"], "mllvm=">, Flags<[HelpHidden]>, Alias<mllvm>;
 
 def dry_run : Flag<["--", "-"], "dry-run">, Flags<[WrapperOnlyOption]>,
   HelpText<"Print generated commands without running.">;